ath9k: implement channel utilization stats for survey

Results for the active channel are updated whenever a new survey dump
is requested, the old data is kept to allow multiple processes to
make their own channel utilization averages.
All other channels only contain the data for the last time that the
hardware was on the channel, i.e. the last scan result or other
off-channel activity.
Running a background scan does not clear the data for the active
channel.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>