Fix type conversion on ID when updating tunnel termination

[mraerino]

Fixes this crash that i was able to reproduce in a regression test:

=== CONT  TestAccNetboxVpnTunnelTermination_basic
panic: interface conversion: interface {} is int, not int64

goroutine 1944 [running]:
github.com/e-breuninger/terraform-provider-netbox/netbox.resourceNetboxVpnTunnelTerminationUpdate(0x14000894c00, {0x105c46d60, 0x14000023290})
        /Users/marcus/src/ffddorf/terraform-provider-netbox/netbox/resource_netbox_vpn_tunnel_termination.go:143 +0x434
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).update(0x105f73320?, {0x105f73320?, 0x14000a80f30?}, 0xd?, {0x105c46d60?, 0x14000023290?})
        /Users/marcus/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:828 +0x130
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).Apply(0x1400027b0a0, {0x105f73320, 0x14000a80f30}, 0x1400093de10, 0x14000894a80, {0x105c46d60, 0x14000023290})
        /Users/marcus/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:947 +0x670
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ApplyResourceChange(0x140006a0000, {0x105f73320?, 0x14000a80ea0?}, 0x1400090d1d0)
        /Users/marcus/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:1153 +0xaa4
github.com/hashicorp/terraform-plugin-go/tfprotov5/tf5server.(*server).ApplyResourceChange(0x140004c8500, {0x105f73320?, 0x14000a80690?}, 0x14000356620)
        /Users/marcus/go/pkg/mod/github.com/hashicorp/[email protected]/tfprotov5/tf5server/server.go:865 +0x2b4
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ApplyResourceChange_Handler({0x105e96660, 0x140004c8500}, {0x105f73320, 0x14000a80690}, 0x14000894680, 0x0)
        /Users/marcus/go/pkg/mod/github.com/hashicorp/[email protected]/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:518 +0x1c0
google.golang.org/grpc.(*Server).processUnaryRPC(0x14000452200, {0x105f73320, 0x14000a80600}, {0x105f79380, 0x14000970480}, 0x1400075f7a0, 0x140008189f0, 0x10700dbb8, 0x0)
        /Users/marcus/go/pkg/mod/google.golang.org/[email protected]/server.go:1369 +0xb58
google.golang.org/grpc.(*Server).handleStream(0x14000452200, {0x105f79380, 0x14000970480}, 0x1400075f7a0)
        /Users/marcus/go/pkg/mod/google.golang.org/[email protected]/server.go:1780 +0xb20
google.golang.org/grpc.(*Server).serveStreams.func2.1()
        /Users/marcus/go/pkg/mod/google.golang.org/[email protected]/server.go:1019 +0x8c
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 1832
        /Users/marcus/go/pkg/mod/google.golang.org/[email protected]/server.go:1030 +0x13c
FAIL    github.com/e-breuninger/terraform-provider-netbox/netbox        5.509s
FAIL