iio: devm_regulator_get_optional never returns NULL

This patch is inspired by a comment of Jonathan Cameron on patch of
Linus Walleij commit aeb55fff ("iio:
st_sensors: fetch and enable regulators unconditionally"). Because
changes made in this patch are actually reference generators they should
be using devm_regulator_get_optional, but if they do not explicitly set
the reference to NULL they should not be using IS_ERR_OR_NULL, but
simple IS_ERR check.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>