regulator: lm363x: Check return value of gpiod_get_index_optional

gpiod_get_index_optional can return ERR_PTR, add IS_ERR checking for it.
While at it, also remove a redundant NULL test for gpiod in error path.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>