ath9k: Only add fix_rssi_inv_only when spectral code is used

The code is only used when ATH9K_DEBUGFS is activated and causes build warnings
when it is still compiled without user.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>