net: rtl8139: do not use old_mmio accesses

Both io and memory use the same mmio functions in the rtl8139 device.
This patch removes the separate MemoryRegionOps and old_mmio accessors
for memory, and replaces it with an alias to the io memory region.

Signed-off-by: Matt Parker <mtparkr@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>