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>