nl80211: Add sta_flags to the station info

Reuse the already existing struct nl80211_sta_flag_update to specify
both, a flag mask and the flag set itself. This means
nl80211_sta_flag_update is now used for setting station flags and also
for getting station flags.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>