ACPI: Do not modify SCI_EN directly

According to the ACPI specification the SCI_EN flag is controlled by
the hardware, which sets this flag to inform the kernel that ACPI is
enabled.  For this reason, we shouldn't try to modify SCI_EN
directly.  Also, we don't need to do it in irqrouter_resume(), since
lower-level resume code takes care of enabling ACPI in case it hasn't
been enabled by the BIOS before passing control to the kernel (which
by the way is against the ACPI specification).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>