USB: pl2303: distinguish between original and cloned HX chips

According to Prolific, several (unauthorized) cheap and less functional
clones of the PL2303HX chip are in circulation. [1]
I've had the chance to test such a cloned device and it turned out that
it doesn't support any baud rates above 115200 baud (original: 6 Mbaud)
It also doesn't support the divisior based baud rate encoding method,
so no continuous baud rate adjustment is possible.
Nevertheless, these devices have been working (unintentionally) with
the driver up to commit 61fa8d69 ("pl2303: also use the divisor based
baud rate encoding method for baud rates < 115200 with HX chips"), and
this commit broke support for them.
Fortunately, it is pretty simple to distinguish between the original
and the cloned HX chips, so I've added a check and an extra chip type
to keep the clones working.
The same check is used by the latest Prolific Windows driver, so it
should be solid.

[1] http://www.prolific.com.tw/US/ShowProduct.aspx?p_id=225&pcid=41



Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>