x86/irq: Clean up VECTOR_UNDEFINED and VECTOR_RETRIGGERED definition

During another patch review, David Rientjes noted that
VECTOR_UNDEFINED and VECTOR_RETRIGGERED should be defined with ()s
so that they are not erroneously used in an arithmetic operation.

Suggested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Yang Zhang <yang.z.zhang@Intel.com>
Link: http://lkml.kernel.org/r/1396440827-18352-1-git-send-email-prarit@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/hw_irq.h

@@ -190,8 +190,8 @@ extern void (*__initconst interrupt[NR_VECTORS-FIRST_EXTERNAL_VECTOR])(void);
 #define trace_interrupt interrupt
 #endif
 
-#define VECTOR_UNDEFINED	-1
-#define VECTOR_RETRIGGERED	-2
+#define VECTOR_UNDEFINED	(-1)
+#define VECTOR_RETRIGGERED	(-2)
 
 typedef int vector_irq_t[NR_VECTORS];
 DECLARE_PER_CPU(vector_irq_t, vector_irq);