Bluetooth: Make LE scan interval and window a controller option

The scan interval and window for LE passive scanning and connection
establishment should be configurable on a per controller basis. So
introduce a setting that later on will allow modifying it.

This setting does not affect LE active scanning during device
discovery phase. As long as that phase uses interleaved discovery,
it will continuously scan.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>