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>