gpio-uclass.c: Fix comparison of unsigned expression warning

We declare that gpio_base (which is the base for counting gpios, not an
address) is unsigned.  Therefore the comparison with >= 0 is always
true.  As the desire is to allow for this base number to be 0, we can
just drop this check.  Reported by clang-3.8.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>