ath6kl: Fix compilation error while compiling w/o debug

drivers/net/wireless/ath/ath6kl/htc_hif.o:
In function `ath6kl_debug_fwlog_event':
drivers/net/wireless/ath/ath6kl/debug.h:109:
multiple definition of `ath6kl_debug_fwlog_event'
drivers/net/wireless/ath/ath6kl/debug.o:
drivers/net/wireless/ath/ath6kl/debug.h:109: first defined here
drivers/net/wireless/ath/ath6kl/htc_hif.o:
In function `ath6kl_debug_cleanup':
drivers/net/wireless/ath/ath6kl/debug.h:118:
multiple definition of `ath6kl_debug_cleanup'
drivers/net/wireless/ath/ath6kl/debug.o:
drivers/net/wireless/ath/ath6kl/debug.h:118: first defined here

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>