mt76: use IEEE80211_OFFLOAD_ENCAP_ENABLED instead of MT_DRV_AMSDU_OFFLOAD

Drop MT_DRV_AMSDU_OFFLOAD after introducing IEEE80211_OFFLOAD_ENCAP_ENABLED
flag in mac80211. Driver now checks vif->offload_flags to know encap offload
status of each interface.

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