wifi: mac80211: fix flow dissection for forwarded packets

Adjust the network header to point at the correct payload offset

Fixes: 986e43b1

 ("wifi: mac80211: fix receiving A-MSDU frames on mesh interfaces")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230324120924.38412-2-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>