Bluetooth: Controller: Fix assertion due to cleared pending radio IRQ

Fix assertion on scan disable observed in BabbleSIM audio
tests due to clearing of pending radio IRQ under race
conditions between scanning being stopped by application
and scanning stopped on central connection setup.

irq_enable/disable is not needed either as radio_isr_set
is always called from Radio or LLL execution context which
are co-operative with each other at same execution priority.

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