[Silabs] Fix test value fallback for hardware version string read

[jmartinez-silabs]

Silabs Device data provider has a fallback to test values when the requested data hasn't been provided to the device.

The fallback for the Hardware version String was using a define that isn't defined anywhere in Matter.

Replace that undefined name with the correct default test value CHIP_DEVICE_CONFIG_DEFAULT_DEVICE_HARDWARE_VERSION_STRING

The size provide in the memcpy was also wrong.

tested on an unprovided EFR32 with chip-tool
chip-tool basicinformation read hardware-version-string $NODE_ID 0
now correctly receives the test value (TEST_VERSION)

[github-actions]

PR #25864: Size comparison from 1f7e4a1 to 5c40f1c

Decreases (1 build for cc32xx)

platform	target	config	section	1f7e4a1	5c40f1cd	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20309907	20309906	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	1f7e4a1	5c40f1cd	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645745	645745	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	301604	301604	0	0.0
			.debug_info	20309907	20309906	-1	-0.0
			.debug_line	2680953	2680953	0	0.0
			.debug_loc	2827596	2827596	0	0.0
			.debug_ranges	286376	286376	0	0.0
			.debug_str	3041114	3041114	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	380555	380555	0	0.0
			.symtab	257456	257456	0	0.0
			.text	537672	537672	0	0.0