fluke-dmm: reduce indentation in 18x/190/28x dispatch logic

fluke-dmm: reduce indentation in 18x/190/28x dispatch logic

Address several style nits in the fluke-dmm driver. Rephrase a lengthy
if-else-if sequence by means of switch-case, so that conditions are
easier to review. Use glib to get the vector length. Handle the trivial
case first, which reduces indentation for the remaining complex logic.
Fixup comment style, break long text lines.