[LinuxCNC/linuxcnc PR#118] Allow specifying of device on wj200_vfd init

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I had the need to use the wj200_vfd module over a USB to RS485 converter, so I’m making this pull request.

This commit adds the –device option to the wj200_vfd HAL component.
It allows the user to select which device to use for Modbus communication
to the VFD. For example, to use a USB to RS485 converter on /dev/ttyUSB0
you would run this command:

loadusr -W wj200_vfd --device /dev/ttyUSB0

The default is /dev/ttyS0 if --device is not specified.

Signed-off-by: James Waples jamwaffles@gmail.com