powerpc/fsl_pci: Check for get_user/probe_kernel_address failure

Signed-off-by: Scott Wood <scottwood@freescale.com>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Hongtao Jia <hongtao.jia@freescale.com>