Bluetooth: Controller: Fix overlapping 1M and Coded PHY scannning

Fix overlapping 1M and Coded PHY scanning that caused idle
radio time when both PHY use same scan interval and sum of
their scan window duration equals the interval.
Implementation now will use continuous scanning and offset
the start of Coded PHY by the window duration of the 1M
scanning.

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