kernel: b53: fix compilation on BCM47XX

This fixes:
drivers/net/phy/b53/b53_priv.h:325:2: error: enumeration value '<board>' not handled in switch [-Werror=switch]
errors.

Fixes: 0de2213eea ("kernel: b53: look for NVRAM's "robo_reset" entry on every platform")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/generic/files/drivers/net/phy/b53/b53_priv.h

@@ -326,6 +326,8 @@ static inline int b53_switch_get_reset_gpio(struct b53_device *dev)
 	case BCM47XX_BOARD_LINKSYS_WRT300NV11:
 	case BCM47XX_BOARD_LINKSYS_WRT310NV1:
 		return 8;
+	default:
+		break;
 	}
 #endif