x86/apic: Do not clear nr_irqs_gsi if no legacy irqs

nr_legacy_irqs is set in probe_nr_irqs_gsi, we should not clear
it after that. Otherwise, the result is that MSI irqs will be
allocated from the wrong range for the systems without legacy
PIC.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>