iio: magnetometer: ak8974: Drop dependency on OF

Nothing in this driver depends on OF firmware so drop the dependency
to remove the false impression such a dependency exists.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220413184502.20998-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>