ath5k: pcu: Add a description for 'band' remove one for 'mode'

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/ath/ath5k/pcu.c:115: warning: Function parameter or member 'band' not described in 'ath5k_hw_get_frame_duration'
 drivers/net/wireless/ath/ath5k/pcu.c:955: warning: Excess function parameter 'mode' description in 'ath5k_hw_pcu_init'

Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Reyk Floeter <reyk@openbsd.org>
Cc: "W. S. Bell" <mentor@madwifi.org>
Cc: Luis Rodriguez <mcgrof@winlab.rutgers.edu>
Cc: Pavel Roskin <proski@gnu.org>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200814113933.1903438-10-lee.jones@linaro.org