mt76x0: trim rx skb to proper length

We need to truncate skb to proper length. This fix below message:

wlan0: associating with AP with corrupt beacon and probe response

Fixes: 9d87d9fad47e ("mt76x0: unify tx/rx datapath with mt76x2u driver")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>