Fix ethernet header length field after stripping the mesh header

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/CT5GNZSK28AI.2K6M69OXM9RW5@syracuse/
Fixes: 986e43b1 ("wifi: mac80211: fix receiving A-MSDU frames on mesh interfaces")
Reported-and-tested-by: Nicolas Escande <nico.escande@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230711115052.68430-1-nbd@nbd.nameSigned-off-by: Jakub Kicinski <kuba@kernel.org>