nl80211: Require auth type for NL80211_CMD_AUTHENTICATE

NL80211_ATTR_AUTH_TYPE is a required parameter for
NL80211_CMD_AUTHENTICATE. We are currently (by chance) defaulting to
open system authentication if the attribute is not specified. It is
better to just reject the invalid command.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>