net: stmmac: fix stmmac_pci_probe failed when CONFIG_HAVE_CLK is selected

When the CONFIG_HAVE_CLK is selected for the system, the stmmac_pci_probe
will fail with dmesg:
[    2.167225] stmmaceth 0000:00:14.6: enabling device (0000 -> 0002)
[    2.178267] stmmaceth 0000:00:14.6: enabling bus mastering
[    2.178436] stmmaceth 0000:00:14.6: irq 24 for MSI/MSI-X
[    2.178703] stmmaceth 0000:00:14.6: stmmac_dvr_probe: warning: cannot
get CSR clock
[    2.186503] stmmac_pci_probe: main driver probe failed
[    2.194003] stmmaceth 0000:00:14.6: disabling bus mastering
[    2.196473] stmmaceth: probe of 0000:00:14.6 failed with error -2

This patch fix the issue by breaking the dependency to devm_clk_get()
as the CSR clock can be obtained at priv->plat->clk_csr from pci driver.
Reported-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Signed-off-by: Kweh, Hock Leong <hock.leong.kweh@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -2786,8 +2786,15 @@ struct stmmac_priv *stmmac_dvr_probe(struct device *device,
 	if (IS_ERR(priv->stmmac_clk)) {
 		dev_warn(priv->device, "%s: warning: cannot get CSR clock\n",
 			 __func__);
-		ret = PTR_ERR(priv->stmmac_clk);
-		goto error_clk_get;
+		/* If failed to obtain stmmac_clk and specific clk_csr value
+		 * is NOT passed from the platform, probe fail.
+		 */
+		if (!priv->plat->clk_csr) {
+			ret = PTR_ERR(priv->stmmac_clk);
+			goto error_clk_get;
+		} else {
+			priv->stmmac_clk = NULL;
+		}
 	}
 	clk_prepare_enable(priv->stmmac_clk);