platform/x86/intel/ifs: Add missing kernel-doc entry

Document the test_num member of struct ifs_data.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/lkml/774fd22a-aaee-758d-8195-77bac783ecbc@infradead.org/