ath10k: improve 11b coex

Some firmware revisions need peer phymode to be
specified as MODE_11B when associating as station
to a 11b AP. Otherwise they can starve other
stations.

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