Bluetooth: Controller: Fix BT_CTLR_LOW_LAT_ULL dependency

Fix BT_CTLR_LOW_LAT_ULL dependency after changes in
commit 5119896c ("Bluetooth: Controller: Fix
BT_CTLR_LOW_LAT_ULL conditional code").

BT_CTLR_LOW_LAT_ULL is independent of BT_CTLR_LOW_LAT, where
the later prevents any ULL execution inside a radio event.

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