nl80211: fix old station flags compatibility

My patch to validate station flags broke compatibility
with the old station flags setting where all flags are
always set at once since it always set the mask as all
possible flags which ended up being rejected later in
the station add/modify code.

Fix by parsing only the current flags in the old flags
attribute -- new applications and new flags should use
(and will now require) the new flags attribute where
the mask is given by the application.

Reported-and-tested-by: Thomas Hilber <ath9k-dev@toh.cx>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>