powerpc/booke64: Fix machine check handler to use the right prolog

Machine check exception handler was using a wrong prolog. Hypervisors like
KVM which are called early from the exception handler rely on the interrupt
source.
Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/exceptions-64e.S

@@ -297,7 +297,7 @@ interrupt_end_book3e:
 
 /* Machine Check Interrupt */
 	START_EXCEPTION(machine_check);
-	CRIT_EXCEPTION_PROLOG(0x200, PROLOG_ADDITION_NONE)
+	MC_EXCEPTION_PROLOG(0x200, PROLOG_ADDITION_NONE)
 //	EXCEPTION_COMMON(0x200, PACA_EXMC, INTS_DISABLE)
 //	bl	special_reg_save_mc
 //	addi	r3,r1,STACK_FRAME_OVERHEAD