Bluetooth: Controller: Fix ticker slot reserved check

Fix ticker slot reserved check to handle deferred execution
of ticker worker where ticks_elapsed is greater than
ticks_slot_previous. In which case, each ticker expiring be
checked for overlap with previous slot reservation.

This fixes:
ASSERTION FAIL [0] @ WEST_TOPDIR/zephyr/subsys/bluetooth/
controller/ll_sw/nordic/lll/lll.c:476
lll_preempt_calc: Actual EVENT_OVERHEAD_START_US = 3906

Flash erase operation exposed the assertion, CPU being
halted deferred the ticker_worker collision resolution and
ticker expiry was not individually checked for overlap with
previous expired ticker reservation.

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