net: ethernet: stmmac: Do not use unreachable() in ipq806x_gmac_probe()

When compiling with clang in certain configurations, an objtool warning
appears:

drivers/net/ethernet/stmicro/stmmac/dwmac-ipq806x.o: warning: objtool:
ipq806x_gmac_probe() falls through to next function phy_modes()

This happens because the unreachable annotation in the third switch
statement is not eliminated. The compiler should know that the first
default case would prevent the second and third from being reached as
the comment notes but sanitizer options can make it harder for the
compiler to reason this out.

Help the compiler out by eliminating the unreachable() annotation and
unifying the default case error handling so that there is no objtool
warning, the meaning of the code stays the same, and there is less
duplication.

Reported-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-ipq806x.c

@@ -289,10 +289,7 @@ static int ipq806x_gmac_probe(struct platform_device *pdev)
 		val &= ~NSS_COMMON_GMAC_CTL_PHY_IFACE_SEL;
 		break;
 	default:
-		dev_err(&pdev->dev, "Unsupported PHY mode: \"%s\"\n",
-			phy_modes(gmac->phy_mode));
-		err = -EINVAL;
-		goto err_remove_config_dt;
+		goto err_unsupported_phy;
 	}
 	regmap_write(gmac->nss_common, NSS_COMMON_GMAC_CTL(gmac->id), val);
 
@@ -309,10 +306,7 @@ static int ipq806x_gmac_probe(struct platform_device *pdev)
 			NSS_COMMON_CLK_SRC_CTRL_OFFSET(gmac->id);
 		break;
 	default:
-		dev_err(&pdev->dev, "Unsupported PHY mode: \"%s\"\n",
-			phy_modes(gmac->phy_mode));
-		err = -EINVAL;
-		goto err_remove_config_dt;
+		goto err_unsupported_phy;
 	}
 	regmap_write(gmac->nss_common, NSS_COMMON_CLK_SRC_CTRL, val);
 
@@ -329,8 +323,7 @@ static int ipq806x_gmac_probe(struct platform_device *pdev)
 				NSS_COMMON_CLK_GATE_GMII_TX_EN(gmac->id);
 		break;
 	default:
-		/* We don't get here; the switch above will have errored out */
-		unreachable();
+		goto err_unsupported_phy;
 	}
 	regmap_write(gmac->nss_common, NSS_COMMON_CLK_GATE, val);
 
@@ -361,6 +354,11 @@ static int ipq806x_gmac_probe(struct platform_device *pdev)
 
 	return 0;
 
+err_unsupported_phy:
+	dev_err(&pdev->dev, "Unsupported PHY mode: \"%s\"\n",
+		phy_modes(gmac->phy_mode));
+	err = -EINVAL;
+
 err_remove_config_dt:
 	stmmac_remove_config_dt(pdev, plat_dat);