serial-dmm: fixup 'conn' vs 'serialcomm' confusion

serial-dmm: fixup 'conn' vs 'serialcomm' confusion

The 'conn' field in the device context and the CONN values in the
declarations of supported DMM models seemed inappropriate. They specify
the communication parameters (UART frame format and bitrate), not the
connection (port name). Adjust the respective identifiers.

Also rephrase the evaluation logic. Instead of checking for the absence
of user specs and optionally assigning a fallback value, just preset
from defaults and override from user specs when present. This simplifies
the logic (eliminates a check).