ath10k: fix mu-mimo rx status reporting

MU-MIMO Rx involves different interpretation of
the VHT-SIG-A compared to SU-MIMO.

The incorrect interpretation led ath10k to report
VHT MCS values greater than 9 which subsequently
prompted mac80211 to drop such frames. This
effectively broke Rx with MU-MIMO in many cases
and manifested with a kernel warning in the log
which looked like this:

  [   14.552520] WARNING: CPU: 2 PID: 0 at net/mac80211/rx.c:3578 ieee80211_rx+0x26c/0x940 [mac80211]()
  [   14.552522] Rate marked as a VHT rate but data is invalid: MCS: 10, NSS: 2
  ... call trace follows ...

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