mac80211: add ieee80211_get_operstate() function

Add ieee80211_get_operstate() function to get the operstate
of the netdevice.

This is needed for drivers that need to know when the interface
is IF_OPER_UP (e.g. wl12xx), and block notifiers can't be used
(e.g. because the interface is already IF_OPER_UP, like after
resuming from suspend)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>