Bluetooth: Controller: Fix BT_RX_STACK_SIZE required

During use of tests/bluetooth/shell application to perform
extended scanning with periodic advertising and broadcast
audio source devices around, it was observed that the
application crashed due to too low BT_RX_STACK_SIZE, hence
increase by 128 bytes.

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