ath10k: clean up set_bitrate_mask handling

The code was a bit convoluted. Clean it up and
prepare for future changes.

While at it this fixes incorrect verification of
'single nss' case when ss2 rates were missing
while ss1 and ss3 were requested resulting in
nss=3 being set:

  iw wlan1 set bitrates legacy-5 ht-mcs-5 0 1 2 3 4 5 6 7 16 17 18 19 20 21 22 23 vht-mcs-5 1:0-9 3:0-9

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>