mac80211: clean up an ieee80211_do_open error path

Eliad's comment prompted me to look closer at
the error paths in ieee80211_do_open() and I
found one that should use the error labels.

Also add a comment about the clear_bit since
in many error cases the bit hasn't been set.

Cc: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>