ath6kl: restart concurrent vifs on failed connect

When an ath6kl STA vif is issued a connect command, the firmware will
disconnect all other beaconing vifs in preparation for a potential
channel switch. The case where the connect fails is currently unhandled,
so if a connection attempt on a STA vif fails and any vifs were waiting
for a new channel, simply restart the concurrent vifs on their previous
channel.

Requires that we start tracking the last issued channel in ar->last_ch,
which is valid since ath6kl only supports 1 channel at a time.

Also clear the beaconing vif's want_ch_switch bit regardless of whether
channel switch succeeds, to stop recommitting the same failed profile.

Signed-off-by: Thomas Pedersen <c_tpeder@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>