This issue can be closed.

Sorry, it took so long to reply...
Finally, our company(Prolific) decided to change the new chip from
"Class FFh (Vendor Specific)" to "Class 02h (Communications and CDC Control)"
In this way, the chip's default support can be backward compatible
with the lower version of the Linux kernel (such as 3.X / 4.X,...)

If there are customers who need RTS/CTS and other flow control in the future.
We(Prolific) will provide patch file to support

Finally, thank you for your kind review patch code.

Charles.