wifi: mt76: mt7921: introduce mt7921_get_mac80211_ops utility routine

Since the fw offload capability check is shared between pci,usb and sdio
devices, move it in common init code and reduce code duplication.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>