net: phy: abort loading yt8511 driver in unsupported modes

While investigating the clang `ge` uninitialized variable report, it was
discovered the default switch would have unintended consequences. Due to
the switch to __phy_modify, the driver would modify the ID values in the
default scenario.

Fix this by promoting the interface mode switch and aborting when the
mode is not a supported RGMII mode.

This prevents the `ge` and `fe` variables from ever being used
uninitialized.

Fixes: 48e8c6f1612b ("net: phy: add driver for Motorcomm yt8511 phy")
Reported-by: kernel test robot <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: Peter Geis <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/phy/motorcomm.c

···
 	int oldpage, ret = 0;
 	unsigned int ge, fe;
 
-	/* set clock mode to 125mhz */
 	oldpage = phy_select_page(phydev, YT8511_EXT_CLK_GATE);
 	if (oldpage < 0)
-		goto err_restore_page;
-
-	ret = __phy_modify(phydev, YT8511_PAGE, 0, YT8511_CLK_125M);
-	if (ret < 0)
 		goto err_restore_page;
 
 	/* set rgmii delay mode */
···
 		ge = YT8511_DELAY_RX | YT8511_DELAY_GE_TX_EN;
 		fe = YT8511_DELAY_FE_TX_EN;
 		break;
-	default: /* leave everything alone in other modes */
-		break;
+	default: /* do not support other modes */
+		ret = -EOPNOTSUPP;
+		goto err_restore_page;
 	}
 
 	ret = __phy_modify(phydev, YT8511_PAGE, (YT8511_DELAY_RX | YT8511_DELAY_GE_TX_EN), ge);
+	if (ret < 0)
+		goto err_restore_page;
+
+	/* set clock mode to 125mhz */
+	ret = __phy_modify(phydev, YT8511_PAGE, 0, YT8511_CLK_125M);
 	if (ret < 0)
 		goto err_restore_page;