iio: adc: ad_sigma_delta: don't overallocate scan buffer

Fix overallocating the size of the scan buffer by converting bits to
bytes. The size is meant to be in bytes, so scanbits needs to be
divided by 8.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250701-iio-adc-ad7173-add-spi-offload-support-v3-1-42abb83e3dac@baylibre.com


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