powerpc/fsl_rio: fix a missing error code

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 <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Alexandre Bounine <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>