wifi: iwlwifi: improve tag handling in iwl_request_firmware

We can remove the intermediary string conversion and use drv->fw_index
in the final snprintf directly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20230127002430.175bfffdf2f5.I7ec7a29b2d93a977cb0a39dbcc7c875032eb14b7@changeid
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>