mt76: mt76x02u: do not set NULL beacons

With current implementation we do not cleanup beacon memory, so is not
needed to call mt76x02_mac_set_beacon() with NULL skb.

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