iio: adc: envelope-detector: Drop dependency on OF

Nothing in this driver depends on OF 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/20220530181313.2789-1-andriy.shevchenko@linux.intel.com


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