mt76: mt76x2: fix multi-interface beacon configuration

If the first virtual interface is a station (or an AP with beacons
temporarily disabled), the beacon of the second interface needs to
occupy hardware beacon slot 0.
For some reason the beacon index was incorrectly masked with the
virtual interface beacon mask, which prevents the secondary
interface from sending beacons unless the first one also does.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>