ath10k: fix tx/rx chainmask init

Firmware reports the number of RF chains so use
that for initialization of supp_{tx,rx}_chainmask
instead of using a macro for 3x3 chips.

This should make tx/rx chainmask reports correct
for chips other than 3x3.

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