net: mcf: check receive buffer size register value

ColdFire Fast Ethernet Controller uses a receive buffer size
register(EMRBR) to hold maximum size of all receive buffers.
It is set by a user before any operation. If it was set to be
zero, ColdFire emulator would go into an infinite loop while
receiving data in mcf_fec_receive. Add check to avoid it.

Reported-by: Wjjzhang <wjjzhang@tencent.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/mcf_fec.c

@@ -393,7 +393,7 @@ static void mcf_fec_write(void *opaque, hwaddr addr,
         s->tx_descriptor = s->etdsr;
         break;
     case 0x188:
-        s->emrbr = value & 0x7f0;
+        s->emrbr = value > 0 ? value & 0x7F0 : 0x7F0;
         break;
     default:
         hw_error("mcf_fec_write Bad address 0x%x\n", (int)addr);