x86/hpet: Check for irq==0 when allocating hpet MSI interrupts

irq == 0 is not a valid irq for a irqdomain MSI allocation, but hpet
code checks only for negative return values.

Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/558447AF.30703@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/hpet.c

@@ -578,7 +578,7 @@ static void hpet_msi_capability_lookup(unsigned int start_timer)
 			continue;
 
 		irq = hpet_assign_irq(hpet_domain, hdev, hdev->num);
-		if (irq < 0)
+		if (irq <= 0)
 			continue;
 
 		sprintf(hdev->name, "hpet%d", i);