pcnet: Restart poll timer on pcnet_start

Just like we call into pcnet_poll_timer on stop, we need to call it on
start to trigger the setup of the poll timer.

Patchworks-ID: 35313
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/pcnet.c

@@ -936,6 +936,7 @@ static void pcnet_start(PCNetState *s)
 
     s->csr[0] &= ~0x0004;       /* clear STOP bit */
     s->csr[0] |= 0x0002;
+    pcnet_poll_timer(s);
 }
 
 static void pcnet_stop(PCNetState *s)