iio: magn: hmc5843: Move struct dev_pm_ops out of header

Having this structure defined static in the header lead to
unnecessary duplication and required additional symbol exports.
Use the EXPORT_NS_SIMPLE_DEV_PM_OPS() to clean this up in the same
fashion as many other drivers do this.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220807184534.1037363-3-jic23@kernel.org