iio: ensure scan index is unique at device register

Having two or more channels with the same positive scan_index field
makes no sense if the device supports buffering.  Prevent this situation
by failing to register such a device.

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>