ath9k: export methods related to ACK timeout estimation

Remove static keyword and export ath9k_hw_setslottime(),
ath9k_hw_set_ack_timeout() and ath9k_hw_set_cts_timeout() in hw.h.
These methods will be used in ACK timeout estimation algorithm (dynack)

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>