adf435x: use .format() for Python string formatting

Rephrase the old style '%' operator string formatting, prefer .format()
calls instead. It is assumed that routine call argument lists are more
readable than optional/conditional tuples are, and not running format
specifiers and literal text into each other helps readability as well,
as do the .format() routine's optionally named references to parameters.

Avoid text formatting which involves concatenation, always work from one
single format instruction instead.