mac80211: check add_chanctx callback before use in ieee80211_reconfig

During testing our mac80211 driver a fatal error occurred which was
signalled to mac80211. Upon performing the reconfiguration of the
device a WARN_ON was triggered. This warning checked the return value
of drv_add_chanctx(). However, this returns -EOPNOTSUPP when the
driver does not provide the callback. As the callback is optional
better check it is defined before calling drv_add_chanctx().

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>