mt76: fix iv and CCMP header insertion

The iv from RXD is only for TKIP_RSC/CCMP_PN/GCMP_PN, and it needs a
check for CCMP header insertion. Move mt76_cipher_type to mt76.h to
reduce duplicated code.

Signed-off-by: Xing Song <xing.song@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>