mt76: mt7921: drop offload_flags overwritten

mt76: mt7921: drop offload_flags overwritten

[ Upstream commit 2363b6a646b65a207345b9a9024dff0eff3fec44 ]

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: e0f9fdda81bd ("mt76: mt7921: add ieee80211_ops")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>