mt76: fix building without CONFIG_LEDS_CLASS

When CONFIG_LEDS_CLASS is disabled, or it is a loadable module while
mt76 is built-in, we run into a link error:

drivers/net/wireless/mediatek/mt76/mac80211.o: In function `mt76_register_device':
mac80211.c:(.text+0xb78): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `devm_of_led_classdev_register'

We don't really need a hard dependency here as the driver can presumably
work just fine without LEDs, so this follows the iwlwifi example and
adds a separate Kconfig option for the LED support, this will be available
whenever it will link, and otherwise the respective code gets left out from
the driver object.

Fixes: 17f1de56 ("mt76: add common code shared between multiple chipsets")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/Kconfig

 config MT76_CORE
 	tristate
 
+config MT76_LEDS
+	bool
+	depends on MT76_CORE
+	depends on LEDS_CLASS=y || MT76_CORE=LEDS_CLASS
+	default y
+
 config MT76_USB
 	tristate
 	depends on MT76_CORE

drivers/net/wireless/mediatek/mt76/mac80211.c

@@ -345,9 +345,11 @@ int mt76_register_device(struct mt76_dev *dev, bool vht,
 	mt76_check_sband(dev, NL80211_BAND_2GHZ);
 	mt76_check_sband(dev, NL80211_BAND_5GHZ);
 
-	ret = mt76_led_init(dev);
-	if (ret)
-		return ret;
+	if (IS_ENABLED(CONFIG_MT76_LEDS)) {
+		ret = mt76_led_init(dev);
+		if (ret)
+			return ret;
+	}
 
 	return ieee80211_register_hw(hw);
 }

drivers/net/wireless/mediatek/mt76/mt76x2/pci_init.c

@@ -507,8 +507,10 @@ int mt76x2_register_device(struct mt76x02_dev *dev)
 	mt76x2_dfs_init_detector(dev);
 
 	/* init led callbacks */
-	dev->mt76.led_cdev.brightness_set = mt76x2_led_set_brightness;
-	dev->mt76.led_cdev.blink_set = mt76x2_led_set_blink;
+	if (IS_ENABLED(CONFIG_MT76_LEDS)) {
+		dev->mt76.led_cdev.brightness_set = mt76x2_led_set_brightness;
+		dev->mt76.led_cdev.blink_set = mt76x2_led_set_blink;
+	}
 
 	ret = mt76_register_device(&dev->mt76, true, mt76x02_rates,
 				   ARRAY_SIZE(mt76x02_rates));