Do not copy beacon skb in mt76x02_mac_set_beacon_enable for usb devices
since it will be done in mt76x02_update_beacon_iter. Moreover squash
mt76x02_mac_set_beacon_enable and __mt76x02_mac_set_beacon_enable since
the latter is run just by mt76x02_mac_set_beacon_enable

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