mac80211: trigger ieee80211_sta_work after opening interface

ieee80211_sta_work is disabled while network interface
is down. Therefore, if you configure wireless parameters
before bringing the interface up, these configurations are
not yet effective and association fails.

A workaround from userspace is calling a command like
'iwconfig wlan0 ap any' after the interface is brought up.

To fix this behaviour, trigger execution of ieee80211_sta_work from
ieee80211_open when in STA or IBSS mode.

Signed-off-by: Jan Niehusmann <jan@gondor.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>