staging:iio: scan_index_timestamp move to iio_dev from buffer

This is just a locally cached value that is device specific (rather
than buffer specific.) Hence it wants to come out of the buffer before
we add multiple buffer support.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>