Stop assuming spans are null-terminated in WiFi driver impls.

[bzbarsky-apple]

Fixes #24271

[github-actions]

PR #25318: Size comparison from d50ad19 to 3b82da4

Full report (3 builds for cc32xx, qpg)

platform	target	config	section	d50ad19	3b82da4	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642681	642681	0	0.0
			(read/write)	203672	203672	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197072	197072	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	930077	930077	0	0.0
			.debug_aranges	87280	87280	0	0.0
			.debug_frame	299816	299816	0	0.0
			.debug_info	20256906	20256906	0	0.0
			.debug_line	2656086	2656086	0	0.0
			.debug_loc	2797535	2797535	0	0.0
			.debug_ranges	281768	281768	0	0.0
			.debug_str	3023369	3023369	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105769	105769	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	377825	377825	0	0.0
			.symtab	256256	256256	0	0.0
			.text	534788	534788	0	0.0
qpg	lighting-app	qpg6105+debug	(read/write)	1151340	1151340	0	0.0
			.bss	99804	99804	0	0.0
			.data	852	852	0	0.0
			.text	598436	598436	0	0.0
	lock-app	qpg6105+debug	(read/write)	1118540	1118540	0	0.0
			.bss	96292	96292	0	0.0
			.data	864	864	0	0.0
			.text	565640	565640	0	0.0