wl12xx: adaptive sched scan dwell times

Set the dwell times for sched scan according to the number
of probe requests which are going to be transmitted.
This should fix the too short dwell time problem which
prevented some of the probe requests from being transmitted
in cases of high number of SSIDs (10+) to be actively sched scanned.
However, in the common case of having up to 1-2 SSIDs that
require active scan, the dwell time would be kept to a minimum
which should conserve power. This is important as sched scan
also runs periodically while the host is suspended and there's
great importance to keep power consumption as low as possible.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
[fixed a couple of new strict checkpatch warnings]
Signed-off-by: Luciano Coelho <coelho@ti.com>