ps3rom: fix error return code

Set the return variable to an error code as done elsewhere in the function.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-By: Geoff Levand <geoff@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>