iio: magnetometer: hmc5843: 'hmc5843_pm_ops' is unused in 1 of 3 files including hmc5843_core.h

We know that it's okay for 'hmc5843_pm_ops' to be unused here.

Fixes the following W=1 kernel build warning(s):

 In file included from include/linux/device.h:25,
 from include/linux/iio/iio.h:10,
 from drivers/iio/magnetometer/hmc5843_core.c:16:
 drivers/iio/magnetometer/hmc5843.h:55:26: warning: ‘hmc5843_pm_ops’ defined but not used [-Wunused-const-variable=]
 55 | static SIMPLE_DEV_PM_OPS(hmc5843_pm_ops,
 | ^~~~~~~~~~~~~~
 include/linux/pm.h:354:25: note: in definition of macro ‘SIMPLE_DEV_PM_OPS’
 354 | const struct dev_pm_ops name = { | ^~~~

Cc: Josef Gajdusek <atx@atx.name>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>