iio: adc: intel_mrfld_adc: Allocating too much data in probe()

This probe function is passing the wrong size to devm_iio_device_alloc().
It is supposed to be the size of the private data.  Fortunately,
sizeof(*indio_dev) is larger than sizeof(struct mrfld_adc) so it doesn't
cause a runtime problem.

Fixes: a7118662 ("iio: adc: intel_mrfld_adc: Add Basin Cove ADC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>