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>

drivers/staging/iio/ring_sw.c

@@ -293,7 +293,7 @@ again:
 		return -EAGAIN;
 	memcpy(data, last_written_p_copy, ring->buf.bpd);
 
-	if (unlikely(ring->last_written_p >= last_written_p_copy))
+	if (unlikely(ring->last_written_p != last_written_p_copy))
 		goto again;
 
 	iio_unmark_sw_rb_in_use(&ring->buf);