mac80211: fix remain-on-channel (non-)cancelling

Felix Liao reported that when an interface is set DOWN
while another interface is executing a ROC, the warning
in ieee80211_start_next_roc() (about the first item on
the list having started already) triggers.

This is because ieee80211_roc_purge() calls it even if
it never actually changed the list of ROC items. To fix
this, simply remove the function call. If it is needed
then it will be done by the ieee80211_sw_roc_work()
function when the ROC item that is being removed while
active is cleaned up.

Cc: stable@vger.kernel.org
Reported-by: Felix Liao <Felix.Liao@watchguard.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>