Staging: iio: fix sizeof *ring should be sizeof(*ring)

Fix checkpatch.pl issues with sizeof *ring should be sizeof(*ring)
in sca3000_ring.c

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>