serial-dmm: Rename VOLTCRAFT_VC820 to VOLTCRAFT_VC820_SER.

serial-dmm: Rename VOLTCRAFT_VC820 to VOLTCRAFT_VC820_SER.

This matches the _ser suffix of the other functions/variables for this
device ("ser" denotes that it is used with the serial UNI-T UT-D02 cable
as opposed the USB/HID based cables which are handled by the uni-t-dmm driver).

But more importantly, the _SER suffix for the enum value will prevent
name clashes later, when the uni-t-dmm driver gets a VOLTCRAFT_VC820 enum.

Do the same for VOLTCRAFT_VC840.