wifi: mt76: mt7915: deal with special variant of mt7916

A variant of mt7916 supports up to 3 tx/rx paths but with only
2 spatial streams. An example usage of the 3rd path is to server as
an auxiliary for beamforming.
In order to deal with this case, this patch reworks some parts to
correctly use paths or streams.

Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>