x86: initialize all fields of mp_irqs[mp_irq_entries]

Commit "x86: make config_irqsrc not MPspec specific" introduced some uses
of uninitialized fields in mp_config_acpi_legacy_irqs(). I need the
following patch to get sched-devel/master to boot.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>