USB: serial: mos7840: add a product ID for the new product

Add a new PID 0x7843 to the driver.
Let the new products be able to set up 3 serial ports with the driver.

Note that this depends on e8603076 ("USB: serial: mos7840: clean up
register handling").

Signed-off-by: JackyChou <jackychou@asix.com.tw>
[ johan: mention dependency in case anyone wants to backport this ]
Signed-off-by: Johan Hovold <johan@kernel.org>