regulator: max8973: make default/unset dvs gpio as invalid gpio

If platform data has dvs-gpio value 0 as default/unset then
make this as invalid gpio so that function gpio_is_valid()
can return false on this case.

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