Bluetooth: controller: Increase RX prio stack size

Due to several changes in the way stacks are calculated, 320 bytes is no
longer enough for the controller-only build. After measuring usages of
up to 320 bytes (locally) and 376 (reported by Ricardo Salveti), the
stack size is increased by 128 bytes, up to 448 bytes.

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