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

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

[ Upstream commit 5a2f15c5a8e017d0951e6dc62aa7b5b634f56881 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>