nl80211: Check that netif_runnin is true in cfg80211 code

We do not want to require all the drivers using cfg80211 to need to do
this or to be prepared to handle these commands when the interface is
down.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>