wifi: cfg80211: factor out bridge tunnel / RFC1042 header check

The same check is done in multiple places, unify it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230213100855.34315-2-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>