x86, irq: don't call mp_config_acpi_gsi() if update_mptable is not enabled

Len expressed concern that the update_mptable feature has
side-effects on the ACPI code.

Make it sure explicitly that the code only ever gets called if
the (default disabled) update_mptable boot quirk option is
disabled.

[ Impact: isolate the update_mptable feature from ACPI code more ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Len Brown <lenb@kernel.org>
LKML-Reference: <4A0DC832.5090200@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>