mac80211: go out of PS before sending disassoc

on disassoc, ieee80211_set_disassoc() goes out of PS
before indicating BSS_CHANGED_ASSOC (not sure why this
is needed, but some drivers might count on the current
behavior).

However, it does it after sending the disassoc
frame, which results in null-data frame being sent
(in order to go out of ps) after we were already sent
the disassoc, which is invalid.

Fix it by going out of ps before sending the disassoc.

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