iio: buffer: Return error if no callback is given

Return error in case no callback is provided to
`iio_channel_get_all_cb()`. There's no point in setting up a buffer-cb
if no callback is provided.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20201121161457.957-3-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>