wifi: rtl8xxxu: Fix the CCK RSSI calculation

The CCK RSSI calculation is incorrect for the RTL8723BU, RTL8192EU,
and RTL8188FU. Add new functions for these chips with code copied from
their vendor drivers. Use the old code only for the RTL8723AU and
RTL8192CU.

I didn't notice any difference in the reported signal strength with my
RTL8188FU, but I didn't look very hard either.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/926c838f-4997-698b-4da9-44582e2af99a@gmail.com