[Silabs]Fix pigweedLogger assert

[jmartinez-silabs]

Wait until the previous send has completed/emptied the buffer before verifying if we have enough space in the send buffer

Tested with the efr32 lighting-app pw-rpc.
Commissioned successfully the device with the rpc console attached and validated the pw command sending and responses

[github-actions]

PR #26093: Size comparison from e1f306f to 7fe26da

Full report (1 build for cc32xx)

platform	target	config	section	e1f306f	7fe26da	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	643081	643081	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933224	933224	0	0.0
			.debug_aranges	87760	87760	0	0.0
			.debug_frame	302028	302028	0	0.0
			.debug_info	20326062	20326062	0	0.0
			.debug_line	2687403	2687403	0	0.0
			.debug_loc	2838361	2838361	0	0.0
			.debug_ranges	288040	288040	0	0.0
			.debug_str	3042379	3042379	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104393	104393	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	377626	377626	0	0.0
			.symtab	256832	256832	0	0.0
			.text	536568	536568	0	0.0