USB: serial: xr: add support for XR21B1411

The single-port XR21B1411 is similar to the XR21B142X type but uses
12-bit registers and 16-bit register addresses, the register requests
are different and are directed at the device rather than interface, and
5 and 6-bit words are not supported.

The register layout is very similar to XR21B142X except that most
registers are offset by 0xc00 (corresponding to a channel index of 12 in
the MSB of wIndex). As the device is single-port so that the derived
channel index is 0, the current register accessors can be reused after
simply changing the address width.

Signed-off-by: Johan Hovold <johan@kernel.org>