ath9k: move RELAY and DEBUG_FS to ATH9K[_HTC]_DEBUGFS

Currently, the common ath9k_common module needs to have a
dependency on RELAY and DEBUG_FS in order to built. This
is usually not a problem. But for RAM and FLASH starved
AR71XX devices, every little bit counts.

This patch adds a new symbol CONFIG_ATH9K_COMMON_DEBUG
which makes it possible to drop the RELAY and DEBUG_FS
dependency there and move it to ATH_(HTC)_DEBUGFS.

Note: The shared FFT/spectral code (which is the only user
of the relayfs in ath9k*) needs DEBUG_FS to export the relayfs
interface to dump the data to userspace. So it makes no sense
to have the functions compiled in, if DEBUG_FS is not there.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>