staging: iio: dummy: fix compile error when not using buffering

Commit 4ae03019 ("staging:iio:dummy: Register same channels for
device and buffer") has changed the number of parameters for
iio_simple_dummy_configure_buffer() only for the IIO_SIMPLE_DUMMY_BUFFER
case.  Fix this by also changing the placeholder function declared in
the header when buffering is not enabled.

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>