ath9k: fix RX_STAT_INC() etc macros

A couple of macros that deal with statistics in ath9k rely on the
declaration of the 'sc' variable, which they dereference.

However, when the statistics are disabled, the new instance in
ath_cmn_process_fft() causes a warning for an unused variable:

drivers/net/wireless/ath/ath9k/common-spectral.c: In function 'ath_cmn_process_fft':
drivers/net/wireless/ath/ath9k/common-spectral.c:474:20: error: unused variable 'sc' [-Werror=unused-variable]

It's better if those macros only operate on their arguments instead of
known variable names, and adding a cast to (void) kills off that warning.

Fixes: 03224678 ("ath9k: add counters for good and errorneous FFT/spectral frames")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>