powerpc/fsl_rio: fix a missing error code

We should set the error code here rather than incorrectly returning 0.
Otherwise static checkers complain.

Link: http://lkml.kernel.org/r/20160804053525.GM775@mwanda


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>