Bluetooth: Controller: Fix incorrect irq priority check

External interrupts are indexed from value 16, wherein
0 to 15 are ARM cortex M exceptions. Fixed code in
_irq_is_priority_equal to fetch correct external
interrupt line ISR priority.

Change-id: I9cfd411480e78dfc9635e72d14df9d667a9d8400
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>