rt2800 olny 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 registred https://dev.archive.openwrt.org/ticket/19081 and
this patch solves the issue.
Signed-off-by: Markov Mikhail <markov.mikhail@itmh.ru>