staging:iio: one more fix for IIO_RING_BUFFER not enabled.

Same problem as seen and fixed in adis16204 et al.
These were fixed by Randy in:
staging: fix more iio builds when IIO_RING_BUFFER is not enabled

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>