iio: magn: ak8975: fix regulator usage

IS_ERR_OR_NULL() should never be used with regulators because
a NULL pointer may be a perfectly valid dummy regulator

We should always succeed to fetch and enable a regulator, but
it may be a dummy. That is fine, so bail out for any real
errors or probe deferrals

Include the error code in the warning print so we know what
kind of problem we're dealing with (for example it is nice to
see if it is a probe deferral).

As we will bail out of probe if the regulator is erroneous,
just issue regulator_disable() on the poweroff path: it will
succeed.

Cc: Mark Brown <broonie@kernel.org>
Cc: Lars-Peter Clausen Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>