wifi: rtl8xxxu: Register the LED and make it blink

If the chip can have an LED, register a struct led_classdev and enable
hardware-controlled blinking. When the chip is not transmitting or
receiving anything the LED is off. Otherwise the LED will blink
faster or slower according to the throughput.

The LED can be controlled from userspace by writing 0, 1, or 2 to
/sys/class/leds/rtl8xxxu-usbX-Y/brightness:
0 - solid off.
1 - solid on.
2 - hardware-controlled blinking.

In this patch none of the chips advertise having an LED. That will be
added in the next patches.

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/b8235bca-60c3-d0fe-a958-53c6dd3ba3f6@gmail.com