mt76: mt76x2: fix multi-interface beacon configuration

mt76: mt76x2: fix multi-interface beacon configuration

commit 5289976ad887deb07c76df7eecf553c264aeebed upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>