acpi: piix4: remove not needed GPE0 mask

Hardcoded GPE0 mask isn't really needed. Since GPE0_STS initialized
with all bits cleared and only QEMU itself can set bits there (i.e.
guest can only clear bits in it). So guest can't triger SCI
by setting _STS & _EN bits and there is not reason to mask out not
supported _STS bits since they shouldn't be set by QEMU in the first
place.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
