Prolific has developed a new USB to UART chip: PL2303HXN
PL2303HXN : PL2303GC/PL2303GS/PL2303GT/PL2303GL/PL2303GE/PL2303GB
The Vendor request used by the PL2303HXN (TYPE_HXN) is different from
the existing PL2303 series (TYPE_HX & TYPE_01).
Therefore, different Vendor requests are used to issue related commands.

1. Added a new TYPE_HXN type in pl2303_type_data, and then executes
   new Vendor request,new flow control and other related instructions
   if TYPE_HXN is recognized.

2. Because the new PL2303HXN only accept the new Vendor request,
   the old Vendor request cannot be accepted (the error message
   will be returned)
   So first determine the TYPE_HX or TYPE_HXN through
   PL2303_READ_TYPE_HX_STATUS in pl2303_startup.

  2.1 If the return message is "1", then the PL2303 is the existing
      TYPE_HX/ TYPE_01 series.
      The other settings in pl2303_startup are to continue execution.
  2.2 If the return message is "not 1", then the PL2303 is the new
      TYPE_HXN series.
      The other settings in pl2303_startup are ignored.
      (PL2303HXN will directly use the default value in the hardware,
       no need to add additional settings through the software)

3. In pl2303_open: Because TYPE_HXN is different from the instruction of reset
   down/up stream used by TYPE_HX.
   Therefore, we will also execute different instructions here.

4. In pl2303_set_termios: The UART flow control instructions used by
   TYPE_HXN/TYPE_HX/TYPE_01 are different.
   Therefore, we will also execute different instructions here.

5. In pl2303_vendor_read & pl2303_vendor_write, since TYPE_HXN is different
   from the vendor request instruction used by TYPE_HX/TYPE_01,
   it will also execute different instructions here.

6. In pl2303_update_reg: TYPE_HXN used different register for flow control.
   Therefore, we will also execute different instructions here.

Signed-off-by: Charles Yeh <charlesyeh522@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>