net: wireless: bcmdhd: Reduce scan dwell time to reduce power

Scan dwell time is reduced for 40/80 to 20/40 (normal)
and 20/30 (power-save). Scan quality reduction is ~ 20%.

b/17485849

Change-Id: I00d6e5ef83df8b075b48736daaa6e814dd09054a
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>