iio: magnetometer: rm3100: Use get_unaligned_be24()

This makes the driver code slightly easier to read.

Cc: Song Qiang <songqiang1304521@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>