Bluetooth: controller: Fix ticker job_guard

Move the ticker job_guard reset to after
ticker_job_compare_update, so that the ticks_current and
ticks_slot_previous are updated before ticker_worker gets
to execute. Without this fix, there is a possibility that
ticker_worker will use incorrect ticks_slot_previous and
ticks_current value under race conditions.

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