Bluetooth: controller: Fix enable and disable of scan state

Updated controller implementation to disallow disabling
initiator state using scan disable. But allow disabling an
already disabled scan state. Also, disallow enabling scan
state while in initiator state.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>