wifi: mt76: mt7996: fix beamformee ss subfield in EHT PHY cap

According to P802.11be_D3.2 Table 9-404m, the minimum value of
Beamformee SS field shall be 3. Fix the values to follow the spec.

Fixes: 348533eb ("wifi: mt76: mt7996: add EHT capability init")
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>