Update CFE_MSG_Message_t conversions to use CFE_MSG_PTR macro

[jphickey]

Is your feature request related to a problem? Please describe.
In nasa/cFE#1966 introduces a CFE_MSG_PTR macro which converts a cmd/tlm header object to a CFE_MSG_Message_t* pointer, which is intended to be used rather than assuming a specific member name (e.g. .Msg).

Describe the solution you'd like
Use the macro instead of assuming .Msg member name.

Additional context
Required when using generated headers, as the member name may not be "Msg" or may be further encapsulated.

Requester Info
Joseph Hickey, Vantage Systems, Inc.