ath10k: clean up monitor start code

Remove useless code that was causing WARN_ON when
a 80MHz+ vif entered promiscuous mode or monitor
interface was started.

The channel mode is already computed by
chan_to_phymode().

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