iio: adc: ad7192: Use sysfs_emit_at

Replace scnprintf with sysfs_emit_at which is the preferred alternative.

Also make sure each fractional digit is in its place by padding with
zeros up to 3 digits: "...%03d...".

Signed-off-by: Alisa Roman <alisa.roman@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230620163135.93780-1-alisa.roman@analog.com


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