ACPI: handle systems which asynchoronously enable ACPI mode

Folklore suggested that such systems existed
in the pre-history of ACPI.

However, we removed the SCI_EN polling loop from
acpi_hw_set_mode() in b430acbd


because it delayed resume by 3 seconds on boxes
that refused to set SCI_EN.

Matthew removed the call to acpi_enable() from
the suspend resume path.

James found a modern system that still needs to be polled
upon boot.

So here we restore the workaround, except that we
put it in acpi_enable() rather than the low level
acpi_hw_set_mode().

https://bugzilla.kernel.org/show_bug.cgi?id=16271

Signed-off-by: Len Brown <len.brown@intel.com>