mt76: mt7921: drop offload_flags overwritten

offload_flags have to be dropped for mt7921 because mt76.omac_idx 0 would
always run as station mode that would cause Tx encapsulation setting
is not applied to mac80211.

Also, drop IEEE80211_OFFLOAD_ENCAP_4ADDR too because it is not really
being supported.

Fixes: e0f9fdda

 ("mt76: mt7921: add ieee80211_ops")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>