ath9k: Build RFKILL feature even when RFKILL subsystem is a MODULE

Currently, ath9k builds RFKILL feature only when the RFKILL subsystem
is built part of the kernel. Build RFKILL feature regardless of whether
RFKILL subsystem is built as a MODULE or part of the kernel.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>