cfg80211: fix Kconfig for users of cfg80211

 * iwm doesn't depend on cfg80211 or wireless extensions
 * rndis wlan selects cfg80211 - needs to depend
 * mac80211 selects cfg80211 - needs to depend

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>