Bluetooth: Controller: Be more explicit with thread names

The "BT RX pri" and "BT RX" are too easily confused with host-side threads,
e.g. the host RX thread is called "BT RX WQ". Add "CTLR" to the Controller
side thread names to make it clear that they're managed by the controller.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>