ath10k: fix channel switching

In the midst of chanctx patch review channel
switching became broken which I failed to notice
until now.

Function ath10k_mac_vif_chan() reports current
chandef which isn't updated until after
switch_vif_chanctx() is returned from.
Consequently the driver just restarted operation
on channels it was residing already instead of
switching to the new ones.

Fixes: 500ff9f9 ("ath10k: implement chanctx API")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>