Bug 583651 – Pin code for Sony CMT-DH5BT too long

Add a way to force the number of digits in the PIN code for a
particular device.

The Sony CMT-DH5BT can enter random pincodes, but with a maximum of
4 digits. Add a quirk for that, and remove the hardcoding of 6 pin
digits (replacing it with a #define in pin.h)