staging: iio: ad7816: Replace sprintf() with sysfs_emit()

staging: iio: ad7816: Replace sprintf() with sysfs_emit()

As stated in Documentation/filesystems/sysfs.rst:
'New implementations of show() methods should only use sysfs_emit() or
sysfs_emit_at() when formatting the value to be returned to user space.'

Replace sprintf with sysfs_emit in the following sysfs show functions:
- ad7816_show_mode()
- ad7816_show_available_modes()
- ad7816_show_channel()
- ad7816_show_value()
- ad7816_show_oti()

Signed-off-by: Bruno Martins <ehanoc@protonmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>