rt2x00: Mark active channel's survey data as "in use"

This is just a cosmetical fix since we only return survey data for the
active channel but it allows iw to show that the survey data is
for the currently used channel.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>