mt76: mac: minor optimizations in mt76x02_mac_tx_rate_val

Do not set bw variable to zero for legacy rates since it is already
initialized to zero. Moreover set nss to 1 just for legacy rates
since nss will be properly set for VHT/HT rates

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>