iio: magnetometer: ak8974: Silence deferred-probe error

It's not uncommon that voltage regulator becomes available later during
kernel's boot process. This patch adds info message about unavailable
regulators in a case of the deferred-probe error and also amends the
error message with a error code.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>