ath9k: fix build error with LEDS_CLASS=m

When CONFIG_ATH9K is built-in but LED support is in a loadable
module, both ath9k drivers fails to link:

x86_64-linux-ld: drivers/net/wireless/ath/ath9k/gpio.o: in function `ath_deinit_leds':
gpio.c:(.text+0x36): undefined reference to `led_classdev_unregister'
x86_64-linux-ld: drivers/net/wireless/ath/ath9k/gpio.o: in function `ath_init_leds':
gpio.c:(.text+0x179): undefined reference to `led_classdev_register_ext'

The problem is that the 'imply' keyword does not enforce any dependency
but is only a weak hint to Kconfig to enable another symbol from a
defconfig file.

Change imply to a 'depends on LEDS_CLASS' that prevents the incorrect
configuration but still allows building the driver without LED support.

The 'select MAC80211_LEDS' is now ensures that the LED support is
actually used if it is present, and the added Kconfig dependency
on MAC80211_LEDS ensures that it cannot be enabled manually when it
has no effect.

Fixes: 197f466e

 ("ath9k_htc: Do not select MAC80211_LEDS by default")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210125113654.2408057-1-arnd@kernel.org