ath9k_hw: fix sparse warnings on ar9003_rtt.c

This fixes these sparse warnings:

  CHECK   drivers/net/wireless/ath/ath9k/ar9003_rtt.c
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:36:6: warning: symbol 'ar9003_hw_rtt_enable' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:41:6: warning: symbol 'ar9003_hw_rtt_disable' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:46:6: warning: symbol 'ar9003_hw_rtt_set_mask' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:52:6: warning: symbol 'ar9003_hw_rtt_force_restore' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:102:6: warning: symbol 'ar9003_hw_rtt_load_hist' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:135:6: warning: symbol 'ar9003_hw_rtt_fill_hist' was not declared. Should it be static?
drivers/net/wireless/ath/ath9k/ar9003_rtt.c:143:6: warning: symbol 'ar9003_hw_rtt_clear_hist' was not declared. Should it be stati

Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>