powerpc/e500: fix breakage with fsl_rio_mcheck_exception

The wrong MCSR bit was being used on e500mc.  MCSR_BUS_RBERR only exists
on e500v1/v2.  Use MCSR_LD on e500mc, and remove all MCSR checking
in fsl_rio_mcheck_exception as we now no longer call that function
if the appropriate bit in MCSR is not set.

If RIO support was enabled at compile-time, but was never probed, just
return from fsl_rio_mcheck_exception rather than dereference a NULL
pointer.

TODO: There is still a remaining, though comparitively minor, issue in
that this recovery mechanism will falsely engage if there's an unrelated
MCSR_LD event at the same time as a RIO error.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 0ff4ab9..6414a0d 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -425,7 +425,7 @@
 	unsigned long reason = mcsr;
 	int recoverable = 1;
 
-	if (reason & MCSR_BUS_RBERR) {
+	if (reason & MCSR_LD) {
 		recoverable = fsl_rio_mcheck_exception(regs);
 		if (recoverable == 1)
 			goto silent_out;