wifi: rtlwifi: Add rtl8192du/sw.c

This contains the new module's entry point.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/1fdc83ac-4a6f-4645-8837-6e38683b6c84@gmail.com