nl80211: Check iftype in cfg80211 code

We do not want to require all the drivers using cfg80211 to need to do
this. In addition, make the error values consistent by using
EOPNOTSUPP instead of semi-random assortment of errno values.

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