nl80211: Validate MFP flag type when parsing STA flags

NL80211_STA_FLAG_MFP was forgotten from sta_flags_policy. The previous
version added the flag due to the loop used in parse_station_flags,
but the proper behavior would be to allow nla_parse_nested() to go
through the policy for all flags.

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