max8903: removes non zero validity checks on gpios.

Prior to this commit a zero gpio was treated as invalid. Whereas
gpio_is_valid() will treat a zero gpio as valid.

This commit removes the confusion and explicitly uses gpio_is_valid()
throughout. Which in turn results in several of the error messages becoming
redundant and thus removed.

Signed-off-by: Chris Lapa <chris@lapa.com.au>

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>