rtlwifi: Add rate section and its related definition and comment

Add comments to make it to be easier to understand, and add compile time
assertions.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>