ath10k: add set_bitrate_mask callback

Add set_bitrate_mask callback. Currently
ath10k HW is limited to handle only single
fixed rate setting or limit number of used
spatial streams.

Example:
iw wlanX set bitrates legacy-5 ht-mcs-5 vht-mcs-5 2:9
will setup VHT, nss=2, mcs=9

iw wlanX set bitrates legacy-5 18 ht-mcs-5 vht-mcs-5
will setup legacy, 18Mbps

iw wlanX set bitrates legacy-5 ht-mcs-5 3 vht-mcs-5
will setup HT, nss=1, mcs=3

iw wlanX set bitrate legacy-5 ht-mcs-5 vht-mcs-5 1:0-9
will setup nss=1

iw wlanX set bitrate legacy-5 ht-mcs-5 vht-mcs-5 1:0-9 2:0-9
will setup nss=2

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>