ath: regd: Provide description for ath_reg_apply_ir_flags's 'reg' param

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

 drivers/net/wireless/ath/regd.c:378: warning: Function parameter or member 'reg' not described in 'ath_reg_apply_ir_flags'

Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.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/20201102112410.1049272-20-lee.jones@linaro.org