wifi: iwlwifi: fix opmode start/stop race

wifi: iwlwifi: fix opmode start/stop race

There's a race when the device is unbound (maybe because the
module is unloaded) while the opmode start hasn't finished yet.
The complete(request_firmware_complete) after the opmode start
was meant (and commented accordingly) to prevent this problem,
but it's not sufficient when the opmode module is loaded after
the firmware load already completed, which happens regularly
now because firmware load doesn't require userspace, unlike
module load.

Fix this by using the existing opmode registration mutex to
protected the start/stop flows against each other properly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230926110319.85951554fed8.I62f20f40d79d0f136fa05e46d7fc16dc437fa3db@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>