wlcore: configure dwell times according to scan type

Allow configuring different dwell times to the different
scan types (regular and scheduled).

Add new configuration entry (dwell_time_dfs) to
conf_scan_settings, in order to allow setting
different values for normal scan and scheduled scan.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>