rtl8139: limit transmission buffer size in c+ mode

The tx buffer would be re-allocated for tx descriptor with big size
and without LS bit set, this would make guest driver could easily let
qemu to allocate unlimited.

In linux host, a glib failure were easy to be triggered:

GLib-ERROR **: gmem.c:176: failed to allocate 18446744071562067968 bytes

This patch fix this by adding a limit. As the spec didn't tell the maximum size
of buffer allowed, stick it to current CP_TX_BUFFER_SIZE (65536).

Changes from V1:

Drop the while statement and s->cplus_txbuffer check.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
