...regardless of the details of the type of aParams.format, SANE_Frame, which
the SANE documentation at <http://sane.alioth.debian.org/html/doc012.html> is
vague about.  (It could, e.g., validly have negative values in an implementation
where SANE_Frame is an enum with additional, negative enumerators besides the
required SAN_FRAME_GRAY...BLUE.)

Before 2687a5ac "-Werror,-Wtautological-compare
with latest clang" the code had explicitly checked for aParams.format < 0.

Change-Id: Ie2a19e67e942841c5a52b862cb427bac77d6ebb2