regulator: max8973: get rid of extra variable for gpio validity

To find that dvs-gpio is valid or not, gpio API gpio_is_valid()
can be directly used instead of intermediate variable.

Removing the extra variable and using the gpio_is_valid().

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>