wl12xx: add split_scan_timeout debugfs file

Add control over split_scan_timeout through
debugfs. Values are in ms while 0 will disable split scan.

Signed-off-by: Eyal Shapira <eyal@wizey.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>