rtlwifi: access skb->data to get C2H data by macro

The format of C2H data is ID(1 byte) + Length(1 byte) + value, and it is
more readable to use macros to access C2H data.

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