Bluetooth: Controller: Fix assert on role stop/abort

Call to ticker_stop/update can fail under the condition
where in a role is being stopped but at the same time it is
preempted by the role event that also uses ticker_stop/
update.

Also if a role closes graceful while it is being stopped,
the radio ISR will process the stop state with no active
role at that instance in time. In this case just reset the
state to none, the role has already been gracefully closed
before this ISR execution. The above applies to aborting a
role event too.

This commit adds code to detect these conditions and
deterministically recover from it.

This commit fixes the assert observed while stopping
advertiser in the Bluetooth sample scan_adv.

Jira: ZEP-1852

Change-id: I51c8d6e212ef43e3526a199cf7b666a79729c732
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>