ath10k: fix ar->rx_channel updating logic

ath10k: fix ar->rx_channel updating logic

Channel contexts aren't iterable until after
they've been added to the driver. The code assumed
otherwise.

This problem could result in:

 * rx_channel being NULL and forcing Rx path to go
   the slow way to get channel on QCA988X,

 * report incorrect channel when running
   multi-channel on QCA61X4 hw2.1,

 * report incorrect channel after AP channel
   switch.

Fixes: 500ff9f9389d ("ath10k: implement chanctx API")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>