rt2x00: save survey for every channel visited

rt2800 only gives you survey for current channel.

Survey-based ACS algorithms are failing to perform their job when working
with rt2800.

Make rt2800 save survey for every channel visited and be able to give away
that information.

There is a bug registered https://dev.archive.openwrt.org/ticket/19081

 and
this patch solves the issue.

Signed-off-by: Markov Mikhail <markov.mikhail@itmh.ru>
sgruszka: remove unused variable
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201104142524.94375-1-stf_xl@wp.pl