drivers/dma/ioat: Remove now-redundant smp_read_barrier_depends()

Now that READ_ONCE() implies smp_read_barrier_depends(), the
__cleanup() and ioat_abort_descs() functions no longer need their
smp_read_barrier_depends() calls, which this commit removes.
It is actually not entirely clear why this driver ever included
smp_read_barrier_depends() given that it appears to be x86-only and
given that smp_read_barrier_depends() has no effect whatsoever except
on DEC Alpha.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: <dmaengine@vger.kernel.org>