wifi: iwlwifi: uefi: check DSM item validity

The first array index is a bitmap indicating which of the
other values are valid. Check that bitmap before returning
a value.

Fixes: fc7214c3 ("wifi: iwlwifi: read DSM functions from UEFI")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220085


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250828095500.59ec52ff865e.I9e11f497a029eb38f481b2c90c43c0935285216d@changeid