rtl8xxxu: move devices supported by rtlwifi under UNTESTED config

There are still four devices which are currently supported both by the new
rtl8xxxu driver and rtlwifi. To not break existing setups enable the support
for these four devices only when CONFIG_RTL8XXXU_UNTESTED is turned on.

Once rtl8xxxu support is found to be good enough the devices can be removed
from rtlwifi and enabled by default in rtl8xxxu.

Reported-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>