mfd: ab8500-debugfs: Cleaning up unnecessary to test, unsigned can't be negative.

Unsigned variable can't be negative so it is unnecessary to test it

This was found using a static code analysis program called cppcheck

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Lee Jones <lee.jones@linaro.org>