wl12xx: add separate config value for DFS dwell time on sched scan

Use a different value for DFS dwell time when performing a scheduled
scan.  Previously we were using the same value as for normal passive
scans.  This adds some flexibility between these two different types
of passive scan.

For now we use 150 TUs for DFS channel dwell time.  This may need to
be fine-tuned in the future.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>