wifi: rtlwifi: Add rtl8192du/rf.{c,h}

These contain one RF configuration function and some functions related
to dual MAC operation.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/36258d72-11fd-4f70-84c6-782972f706ed@gmail.com