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

[ Upstream commit 9f718554 ]

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>
Stable-dep-of: 9ad79748 ("wifi: cfg80211: check A-MSDU format more carefully")
Signed-off-by: Sasha Levin <sashal@kernel.org>