iio: core: Use sysfs_emit() (trivial bits)

iio: core: Use sysfs_emit() (trivial bits)

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.

This patch converts the places in the iio core that follow the pattern of

   return s*printf(...)

to

   return sysfs_emit(...)

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210320071405.9347-2-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>