iio: __iio_format_value(): Convert to sysfs_emit_at()

iio: __iio_format_value(): 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 __iio_format_value() and related functions to use this new
interface.

This conversion involves changing the signature of __iio_format_value() so
that it similar to sysfs_emit_at() and takes the buffers start address and
an offset where to write within the buffer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210320071405.9347-4-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>