Bluetooth: controller: Fix conn update assert in slave role

Explicitly track the connection update related ticker stop
and start to avoid asserting due to ticker update being done
at the same time for compensating the clock drift.

The compensation related ticker update failure in this case
can be safely ignored as new anchor point is used anyway
at the instant of the connection update.

Fixes #8796

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