ath10k: implement get_survey()

This implements a limited subset of what can be
reported in the survey dump.

This can be used for assessing approximate channel
load, e.g. for automatic channel selection.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>