gpio: Check if base is positive before calling gpio_is_valid()

It doesn't make much sense to make some (possible expensive) calls to
gpio_is_valid() first, and to ignore the result if the base number is
negative. Check for a positive base number first.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>