Bluetooth: Controller: Fix mayfly priority check when in ZLI

Fix mayfly_prio_is_equal() function when BT_CTLR_ZLI is
enabled.

When Zero Latency IRQs are used, LLL execution priority
is the Zero Latency IRQ priority and will not be equal
to ULL execution priority.

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