mac80211: fix IEEE80211_SDATA_DISCONNECT_RESUME

Since commit 12e7f517,
IEEE80211_SDATA_DISCONNECT_RESUME no longer worked as
it would simply never be tested. Restore a bit of the
code removed there and in 9b7d72c1
to make it work again.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>