wifi: iwlwifi: fw: don't use constant size with efi.get_variable

Use efi.get_variable() with NULL pointer for data in order to
obtain entry size and then call it again with the correct size
to get the entry itself.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230606103519.ef95a8055a50.Iae5389baaf0a9a3c89469f7502275ee119d378b6@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>