iio: backend: print message in case op is not implemented

iio: backend: print message in case op is not implemented

For APIs that have a return value, -EOPNOTSUPP is returned in case the
backend does not support the functionality. However, for APIs that do
not have a return value we are left in silence. Hence, at least print a
debug message in case the callback is not implemented by the backend.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240709-dev-iio-backend-add-debugfs-v1-2-fb4b8f2373c7@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>