regulator: gpio-regulator: fix forgotten gpios-states reading

Commit 934624d6 ("regulator: gpio-regulator: do not open-code counting
and access of dt array elements") forgot to convert the recently added
gpios-states property using the same pattern.

Convert this instance to use the of-helpers too, resolving the build error.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bqreaders.com>
Signed-off-by: Mark Brown <broonie@linaro.org>