mac80211: don't allow registering the same rate control twice

Previously, mac80211 would allow registering the same rate control
algorithm twice. This is a programming error in the registration
and should not happen; additionally the second version could never
be selected. Disallow this and warn about it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>