iwlagn: make iwlagn_wait_notification return error code

We're unlikely to care about the actual time spent
waiting, so make the function return an error code
which is less error prone in coding new uses.

Also, while at it, mark __must_check.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>