net: phy: avoid undefined behavior in *_led_polarity_set()

gcc runs into undefined behavior at the end of the three led_polarity_set()
callback functions if it were called with a zero 'modes' argument and it
just ends the function there without returning from it.

This gets flagged by 'objtool' as a function that continues on
to the next one:

drivers/net/phy/aquantia/aquantia_leds.o: warning: objtool: aqr_phy_led_polarity_set+0xf: can't find jump dest instruction at .text+0x5d9
drivers/net/phy/intel-xway.o: warning: objtool: xway_gphy_led_polarity_set() falls through to next function xway_gphy_config_init()
drivers/net/phy/mxl-gpy.o: warning: objtool: gpy_led_polarity_set() falls through to next function gpy_led_hw_control_get()

There is no point to micro-optimize the behavior here to save a single-digit
number of bytes in the kernel, so just change this to a "return -EINVAL"
as we do when any unexpected bits are set.

Fixes: 1758af4 ("net: phy: intel-xway: add support for PHY LEDs")
Fixes: 9d55e68 ("net: phy: aquantia: correctly describe LED polarity override")
Fixes: eb89c79 ("net: phy: mxl-gpy: correctly describe LED polarity")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241217081056.238792-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: arndb

drivers/net/phy/aquantia/aquantia_leds.c

@@ -156,5 +156,5 @@ int aqr_phy_led_polarity_set(struct phy_device *phydev, int index, unsigned long
 	if (force_active_high || force_active_low)
 		return aqr_phy_led_active_low_set(phydev, index, force_active_low);
 
-	unreachable();
+	return -EINVAL;
 }

drivers/net/phy/intel-xway.c

@@ -529,7 +529,7 @@ static int xway_gphy_led_polarity_set(struct phy_device *phydev, int index,
 	if (force_active_high)
 		return phy_clear_bits(phydev, XWAY_MDIO_LED, XWAY_GPHY_LED_INV(index));
 
-	unreachable();
+	return -EINVAL;
 }
 
 static struct phy_driver xway_gphy[] = {

drivers/net/phy/mxl-gpy.c

@@ -1014,7 +1014,7 @@ static int gpy_led_polarity_set(struct phy_device *phydev, int index,
 	if (force_active_high)
 		return phy_clear_bits(phydev, PHY_LED, PHY_LED_POLARITY(index));
 
-	unreachable();
+	return -EINVAL;
 }
 
 static struct phy_driver gpy_drivers[] = {