Bluetooth: controller: Make RX prio thread stack configurable

Now that CONFIG_BT_CTLR_RX_PRIO_STACK_SIZE defines a stack size, allow
for it to be configurable from a .conf file just like the rest of the
threads in Bluetooth.

Fixes #27768.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>