mac80211: don't start the dynamic ps timer if not associated

When we are disconnecting, we set PS off, but this happens before we
send the deauth/disassoc request.  When the deauth/disassoc frames are
sent, we trigger the dynamic ps timer, which then times out and turns
PS back on.  Thus, PS remains on after disconnecting, causing problems
when associating again.

This can be fixed by preventing the timer to start when we're not
associated anymore.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>