iio: common: scmi_iio: use sysfs_emit() to instead of scnprintf()

Replace the open-code with sysfs_emit() to simplify the code.

Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Link: https://lore.kernel.org/r/202212011156314630626@zte.com.cn


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