iio: core: make use of simple_write_to_buffer()

Instead of open coding (kind of) simple_write_to_buffer(), use it.

While at it, use ascii representation to terminate the string as that is
the more common way of doing it.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250218-dev-iio-misc-v1-2-bf72b20a1eb8@analog.com


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