[Silabs] Fix blocking uart

[jepenven-silabs]

Using Event group for notification caused an overflow of the FreeRTOS timer queue, thus leaving the TX task waiting forever. Replaced it with a task notification since the task is only waiting on a single TX callback and nothing else.

[github-actions]

PR #25919: Size comparison from 62f2bb4 to 17da60e

Increases (1 build for cc32xx)

platform	target	config	section	62f2bb4	17da60e	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20310330	20310332	2	0.0

Full report (1 build for cc32xx)

platform	target	config	section	62f2bb4	17da60e	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645825	645825	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	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301596	301596	0	0.0
			.debug_info	20310330	20310332	2	0.0
			.debug_line	2681125	2681125	0	0.0
			.debug_loc	2827568	2827568	0	0.0
			.debug_ranges	286448	286448	0	0.0
			.debug_str	3041144	3041144	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	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	380543	380543	0	0.0
			.symtab	257472	257472	0	0.0
			.text	537752	537752	0	0.0