The original code is available for download from:

http://www.wch.cn/download/CH341SER_LINUX_ZIP.html

However, the code does not build since this commit and another commit that cleaned up wait_interrupt:

commit 1143832eca8f1d64da7d85642c956ae9d25c69e1
Author: Greg Kroah-Hartman <[email protected]>
Date:   Thu Jun 6 10:32:00 2013 -0700

    USB: serial: ports: add minor and port number

and another set of commits that dropped the use of
interruptible_sleep_on since it is racy.  Here's one such commit:

commit 106fd892bc714a9b7c28daba98a3623a41c32f1a
Author: Arnd Bergmann <[email protected]>
Date:   Wed Feb 26 12:01:44 2014 +0100

    swim3: fix interruptible_sleep_on race

I have patched the code for these changes and verified that it works correctly on 4.x kernels for my RGUNO, which is an Indian clone of the Arduino UNO with the CH341 chip. My patches (past, present and future) to this code are licensed as GPLv2 (to match the kernel) but I have no idea what the original code is licensed as, since there is no indication in the code or the website.