mt76: mt7615: fix using VHT STBC rates

The hardware expects MT_TX_RATE_NSS to be filled with the number of
space/time streams. For non-STBC rates, this is equal to nss.
For 1-stream STBC, this needs to be set to 2.
This is relevant for VHT rates only, on HT, the value is derived from MCS
internally.

Signed-off-by: Felix Fietkau <nbd@nbd.name>