mt76: move mt76_insert_ccmp_hdr in mt76-module

Move mt7615_insert_ccmp_hdr in mac80211.c and rename it in
mt76_insert_ccmp_hdr since it is shared between mt7603 and mt7615
drivers

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>