nl80211: Add more through validation of MLME command parameters

Check that the used authentication type and reason code are valid here
so that drivers/mac80211 do not need to care about this. In addition,
remove the unnecessary validation of SSID attribute length which is
taken care of by netlink policy.

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