Bluetooth: controller: Fix ULL_HIGH priority to be lower than LLL

Fix the default ULL_HIGH execution priority to be lower than
LLL execution priority to reduce Radio ISR service latency.

Also, default to Zero Latency IRQ for Radio ISR if enabled
in the kernel.

Fixes #29994.

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