ath10k: simplify computation of mgmt rx band

Using global channel won't work with chanctx. Try
to determine the channel from the information
provided in the wmi event itself alone. This
should be sufficient.

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