staging: iio: ring_sw: Fix incorrect test on successful read of last value, causes infinite loop

This is a bad one. The test means that almost no reads of the last
value ever succeed!  Result is an infinite loop.

Another one for the 'oops' category.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>