gpio: gpiolib: don't compare an unsigned for >= 0

The parameter offset is an unsigned, so it makes no sense to compare
it for >= 0. Fix the compiler warning regarding this by removing this
comparison.

As the macro GPIO_OFFSET_VALID is only used at this single place, simplify
the code by dropping the macro completely and dropping the invert, too.

No functional change.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>