tools: fix gcc sign-compare warnings

tools: fix gcc sign-compare warnings

Cosmetic change to avoid multiple gcc sign-compare warnings between
signed int argc and unsigned int arg_num, both being small and
non-negative. Alternatively, we could use unsigned int argc or
disable the warning with gcc -Wno-sign-compare.

Signed-off-by: Georg Schiesser <georg.schiesser@opentech.at>
[Jan: reordered lines for visual pleasure]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>