net: stmmac: Fix "Unbalanced pm_runtime_enable!" warning

If the device is PCI based like intel-eth-pci, pm_runtime_enable() is
already called by pci_pm_init().

So only pm_runtime_enable() when it's not already enabled.
Signed-off-by: NKai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -7159,7 +7159,8 @@ int stmmac_dvr_probe(struct device *device,
 
 	pm_runtime_get_noresume(device);
 	pm_runtime_set_active(device);
-	pm_runtime_enable(device);
+	if (!pm_runtime_enabled(device))
+		pm_runtime_enable(device);
 
 	if (priv->hw->pcs != STMMAC_PCS_TBI &&
 	    priv->hw->pcs != STMMAC_PCS_RTBI) {