ath9k: Enforce module build if rfkill is a module

CONFIG_ATH9K=y results in build issues if CONFIG_RFKILL=m since ath9k
does not depend on rfkill in kconfig (i.e., CONFIG_RFKILL is used to
select whether to enable rfkill in ath9k), but uses its functions if
rfkill is enabled. Enforce ath9k to be build as a module if
CONFIG_RFKILL=m to avoid this invalid configuration.
Signed-off-by: NJouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/Kconfig

 config ATH9K
 	tristate "Atheros 802.11n wireless cards support"
 	depends on PCI && MAC80211 && WLAN_80211
+	depends on RFKILL || RFKILL=n
 	select MAC80211_LEDS
 	select LEDS_CLASS
 	select NEW_LEDS