mt76: fix LED link time failure

mt76: fix LED link time failure

The mt76_led_cleanup() function is called unconditionally, which
leads to a link error when CONFIG_LEDS is a loadable module or
disabled but mt76 is built-in:

drivers/net/wireless/mediatek/mt76/mac80211.o: In function `mt76_unregister_device':
mac80211.c:(.text+0x2ac): undefined reference to `led_classdev_unregister'

Use the same trick that is guarding the registration, using an
IS_ENABLED() check for the CONFIG_MT76_LEDS symbol that indicates
whether LEDs can be used or not.

Fixes: 36f7e2b2bb1d ("mt76: do not use devm API for led classdev")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>