wifi: iwlwifi: fw: fix compiler warning for NULL string print

When the system is compiled without CONFIG_DMI, the function
here statically returns NULL, leading to a compiler warning.
Catch that and print "<unknown>" in that case.

While at it, fix some indentation in the function.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402030641.zUTuACYV-lkp@intel.com/


Fixes: 09059c67 ("wifi: iwlwifi: prepare for reading PPAG table from UEFI")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>