Bluetooth: controller: Fix BT_CTLR_ULL_HIGH_PRIO default value

Fix BT_CTLR_ULL_HIGH_PRIO default value to equal
BT_CTLR_LLL_PRIO value if BT_CTLR_LOW_LAT is enabled, else
default ULL_HIGH to lower IRQ priority level in comparison
to LLL IRQ priority level. If BT_CTLR_ZLI is enabled,
BT_CTLR_ULL_HIGH_PRIO can default to a priority level value
of 0 and still be lower priority level than the LLL which
is placed at Zero Latency IRQ priority level (highest
priority in the CPU).

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