rtl8139: do the network/host communication only in normal operating mode

According the spec, the card works in network/host communication mode only when
both EEM1 and EEM0 are unset in 93C46 Command Register (normal op
mode). So this patch check these bits before trying to receive packets.

As some guest driver (such as linux, see cp_init_hw() in 8139cp.c)
allocate rx ring after the recevier were enabled, this would cause our
emulation codes tries to dma into guest memory when the rx descriptor
is not properly configured. This patch fixes this.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
