net: macb: ensure the device is available before accessing GEMGXL control registers

If runtime power menagement is enabled, the gigabit ethernet PLL would
be disabled after macb_probe(). During this period of time, the system
would hang up if we try to access GEMGXL control registers.

We can't put runtime_pm_get/runtime_pm_put/ there due to the issue of
sleep inside atomic section (7fa2955f ("sh_eth: Fix sleeping
function called from invalid context"). Add netif_running checking to
ensure the device is available before accessing GEMGXL device.

Changed in v2:
 - Use netif_running instead of its own flag
Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb_main.c

@@ -2867,6 +2867,9 @@ static struct net_device_stats *gem_get_stats(struct macb *bp)
 	struct gem_stats *hwstat = &bp->hw_stats.gem;
 	struct net_device_stats *nstat = &bp->dev->stats;
 
+	if (!netif_running(bp->dev))
+		return nstat;
+
 	gem_update_stats(bp);
 
 	nstat->rx_errors = (hwstat->rx_frame_check_sequence_errors +