mac80211: fix double-start of remain-on-channel

When a remain-on-channel item is deleted, we remove it
from the list and then start the next item. However,
if it wasn't actually the first item then calling
ieee80211_start_next_roc() is wrong as it will start
the first item -- even if that was already started.

Fix the two places that do this and add a warning to
prevent the problem from reoccurring.

Reported-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>