mt76x02: make beacon slots bigger for USB

Since we sent PS buffered frames via beacon memory we need to make
beacon slots bigger. That imply we will also need to decrease number
of slots as beacon SRAM memory is limited to 8kB.

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