wifi: rtl8xxxu: Add rate control code for RTL8188EU

Copied from the newer vendor driver, v5.2.2.4.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/5acc1e5d-62d6-3a6a-0f9e-cbc8b809b1d7@gmail.com