iio: iio_enum_available_read(): Convert to sysfs_emit_at()

iio: iio_enum_available_read(): Convert to sysfs_emit_at()

sysfs_emit() is preferred over raw s*printf() for sysfs attributes since it
knows about the sysfs buffer specifics and has some built-in sanity checks.

Convert the iio_enum_available_read() function to use sysfs_emit_at()
instead of scnprintf().

The conversion is straight forward, the only difference is that
sysfs_emit_at() takes the buffers start address and an offset as parameters
and already knows about the buffer's size limit.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210320071405.9347-3-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>