Add -ECANCELED on return from lll_preempt_calc so that
assert check can be placed in the prepare callbacks of the
state/role implementations.

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>

Move the EVENT_OVERHEAD_START_US verbose assertion to each
state/role LLL implementation so that correct state/role
that is delayed is conveyed in the assertion message.

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>

Fix Connected ISO implementation to correctly handle SN,
NESN and payload_count when prepare callback is aborted due
to CPU overhead related latencies.

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>

Add LL_ASSERT_OVERHEAD define to reuse the assertion check
related to increase in actual EVENT_OVERHEAD_START_US value.

Introduce Kconfig option to permit radio event be skipped
when increased prepare callback latencies are measured,
instead of assertion.

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>