cfg80211: remove channel_switch combination check

Driver is now responsible for veryfing if the
switch is possible.

Since this is inherently tricky driver may decide
to disconnect an interface later with
cfg80211_stop_iface().

This doesn't mean driver can accept everything. It
should do it's best to verify requests and reject
them as soon as possible.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>