iio: buffer: Return error if no callback is given

mainline inclusion
from mainline-v5.11-rc1
commit 6d74a3ee
category: bugfix
bugzilla: 185695 https://gitee.com/openeuler/kernel/issues/I4DDEL



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>
Signed-off-by: tanghui <tanghui20@huawei.com>
Reviewed-by: Zhang Qiao <zhangqiao22@huawei.com>
Reviewed-by: Chen Hui <judy.chenhui@huawei.com>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>