usb/serial/ch341: Add parity support

Based on wireshark packet traces from a windows machine.

ch340 and ch341 both seem to support all parity modes, but only the ch341
appears to support variable data bits and variable stop bits, so those are left
unimplemented, as before.

Tested on a generic usb-rs485 dongle with the chip label scratched off, and
some Modbus/RTU devices that required various parity settings.

Signed-off-by: Karl Palsson <karlp@tweak.net.au>