mt76: mt76x02: minor mt76x02_mac_set_beacon optimization

We do not call mt76x02_mac_set_beacon() with NULL skb any longer and
we do not need to return error value.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>