Bluetooth: Controller: Offset Coded PHY scan window

Offset the Coded PHY scan window by 1M PHY scan window
duration so that when 1M PHY scan window is less than
scan interval then Coded PHY scan window can utilize the
radio thereafter for its scan window duration. For
example, with 20 ms interval and 10 ms window there can
be 10 ms 1M PHY followed by 10 ms of Coded PHY scanning.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>