mt76: mt7915: fix he_mcs capabilities for 160mhz

At 160, this chip can only do 2x2 NSS.  Fix the features
accordingly, verified it shows up properly in iw phy foo info
now.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>