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: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>