wifi: rtw88: Add support for LED blinking

wifi: rtw88: Add support for LED blinking

Register a struct led_classdev with the kernel's LED subsystem and
create a throughput-based trigger for it. Then mac80211 makes the LED
blink.

Tested with Tenda U12 (RTL8812AU), Tenda U9 (RTL8811CU), TP-Link Archer
T2U Nano (RTL8811AU), TP-Link Archer T3U Plus (RTL8812BU), Edimax
EW-7611UCB (RTL8821AU), LM842 (RTL8822CU).

Also tested with devices which don't have LEDs: the laptop's internal
RTL8822CE and a no-name RTL8723DU.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/6c43451f-ab2f-4e76-ac6e-ff5a18dd981d@gmail.com