Do not hardcode beamformee 4 space-time streams support, because
DBDC can only support up to 2.

Fixes: 00b2e16e ("mt76: mt7915: add TxBF capabilities")
Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Felix Fietkau <nbd@nbd.name>