USB: serial: xr: add support for XR21V1412 and XR21V1414

Add support for the two- and four-port variants of XR21V1410.

Use the interface number of each control interface (e.g. 0, 2, 4, 6) to
derive the zero-based channel index:

	XR21V1410	0
	XR21V1412	0, 1
	XR21V1414	0, 1, 2, 3

Note that the UART registers reside in separate blocks per channel,
while the UART Manager functionality is implemented using per-channel
registers.

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