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: 09059c6764 ("wifi: iwlwifi: prepare for reading PPAG table from UEFI")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/intel/iwlwifi/fw/regulatory.c

@@ -397,9 +397,9 @@ bool iwl_is_ppag_approved(struct iwl_fw_runtime *fwrt)
 	if (!dmi_check_system(dmi_ppag_approved_list)) {
 		IWL_DEBUG_RADIO(fwrt,
 				"System vendor '%s' is not in the approved list, disabling PPAG.\n",
-				dmi_get_system_info(DMI_SYS_VENDOR));
-				fwrt->ppag_flags = 0;
-				return false;
+				dmi_get_system_info(DMI_SYS_VENDOR) ?: "<unknown>");
+		fwrt->ppag_flags = 0;
+		return false;
 	}
 
 	return true;