wifi: rtl8xxxu: Add LED control code for RTL8188EU

By default the LED will blink when there is some activity.

This was tested with a TP-Link TL-WN725N.

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/34d62bf5-3595-0c77-2ca7-be78555f765d@gmail.com