i.MX: Fix FEC/ENET receive funtions

The actual imx_eth_enable_rx() function is buggy.

It updates s->regs[ENET_RDAR] after calling qemu_flush_queued_packets().

qemu_flush_queued_packets() is going to call imx_XXX_receive() which itself
is going to call imx_eth_enable_rx().

By updating s->regs[ENET_RDAR] after calling qemu_flush_queued_packets()
we end up updating the register with an outdated value which might
lead to disabling the receive function in the i.MX FEC/ENET device.

This patch change the place where the register update is done so that the
register value stays up to date and the receive function can keep
running.

Reported-by: Fyleo <fyleo45@gmail.com>
Tested-by: Fyleo  <fyleo45@gmail.com>
Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 20180113113445.2705-1-jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Tested-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/net/imx_fec.c

@@ -595,19 +595,16 @@ static void imx_eth_do_tx(IMXFECState *s, uint32_t index)
 static void imx_eth_enable_rx(IMXFECState *s, bool flush)
 {
     IMXFECBufDesc bd;
-    bool rx_ring_full;
 
     imx_fec_read_bd(&bd, s->rx_descriptor);
 
-    rx_ring_full = !(bd.flags & ENET_BD_E);
+    s->regs[ENET_RDAR] = (bd.flags & ENET_BD_E) ? ENET_RDAR_RDAR : 0;
 
-    if (rx_ring_full) {
+    if (!s->regs[ENET_RDAR]) {
         FEC_PRINTF("RX buffer full\n");
     } else if (flush) {
         qemu_flush_queued_packets(qemu_get_queue(s->nic));
     }
-
-    s->regs[ENET_RDAR] = rx_ring_full ? 0 : ENET_RDAR_RDAR;
 }
 
 static void imx_eth_reset(DeviceState *d)
@@ -866,7 +863,6 @@ static void imx_eth_write(void *opaque, hwaddr offset, uint64_t value,
     case ENET_RDAR:
         if (s->regs[ENET_ECR] & ENET_ECR_ETHEREN) {
             if (!s->regs[index]) {
-                s->regs[index] = ENET_RDAR_RDAR;
                 imx_eth_enable_rx(s, true);
             }
         } else {