brymen-dmm: style nits, apply comm param defaults, low battery warning

Rephrase how the default serial communication parameters get applied.
Preset the variable to the default value and let optional user provided
specs override these. This avoids an extra check which is difficult to
read and highly redundant. Add a comment for raised awareness that a
default port spec is undesirable because it's unreliable.

Raise the severity of low battery messages. It's worth warning the user
because measurements could be inaccurate.

Reduce indentation in continuation lines of a long routine signature,
and drop an empty line in a short struct while we are here.