Bluetooth: Controller: Run all enqueued mayfly before disable

Controller asserted in preparation of a role event due to
the previous (same or another) role event preparation being
not complete.

Mayfly callee was disabled in the previous event due to the
preparation time being short and previous start running
(higher natural priority) before all previous preparation
mayfly completed. The previous start disabled mayfly to
avoid Radio ISR latencies.

The current role event that asserted, preempted the
previous role (observer role with continuous scanning
window) which runs until preemption to maximise the Radio
h/w use (observer scanning until next interval). The
previous preparation mayfly is still disabled when the
current role preparation tries to use same mayfly instance
which should be free for a new enqueue.

This commit updates mayfly implementation so that mayfly
callee is disabled only after all enqueued mayfly instances
are run to completion.

Jira: ZEP-1839

Change-id: I3e0d31422db8e47b819189110b11ebd07dd09a7c
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>