wl1271: Optimize scan duration

wl1271: Optimize scan duration

Currently then dwell times for each channel in scans is set to an overly
long value, and excessive number of probe-requests are transmitted on each
channel (for active scans.)

Based on testing, comparable results can be received with smaller dwell-time,
and, with fever probe-requests - in fact, reducing the number of probe-requests
to 2 seems to increase the number of found results.

Configure more optimal values for per channel dwell times. Comparison for the
different scan configurations (in my current office environment):

dwell-time 60000   3x probe-req == ~60 results
           40000   3x probe-req == ~50 results
           30000   3x probe-req == ~40 results

dwell-time 60000   2x probe-req == ~70 results
           40000   2x probe-req == ~60 results
           30000   2x probe-req == ~58 results

The above are results for a cumulative 3 scan run. For individual scans, the
number of results drop slightly more.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>