Revert "mt76: use IEEE80211_OFFLOAD_ENCAP_ENABLED instead of MT_DRV_AMSDU_OFFLOAD"

Revert "mt76: use IEEE80211_OFFLOAD_ENCAP_ENABLED instead of MT_DRV_AMSDU_OFFLOAD"

This reverts commit f17f4864504d754bcbf31e4c89412cdf9946c409 and adds the
MT_DRV_AMSDU_OFFLOAD flag for MT7921 USB/SDIO

The reverted commit significantly decreases performance when running
a test where two MT7915 radios have 16 station vdevs each, configured
for AC mode, and transmitting UDP traffic to AP.

Co-developed-by: Felix Fietkau <nbd@nbd.name>
Reported-by: Carson Vandegriffe <carson.vandegriffe@candelatech.com>
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>