staging:iio: remove userspace access to bytes per datum.

There are no known reasons why userspace should want this value.
It can be established from the buffer description anyway.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>