cfg80211: simplify strict custom alpha2 regdomain check

This makes it easier to read.

Cc: smihir@qti.qualcomm.com
Cc: tushnimb@qca.qualcomm.com
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>