Bluetooth: Controller: Reduce successive ticker_job() on Extended Scan

Reduce number of successive calls to ticker_job() by
disabling mayflies in the ULL_HIGH priority, enqueue
ticker_yield_abs() and ticker_start() before re-enabling
so that single ticker_job() handles both yield and start.

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