Fix for PL2303HX chips so that PL2303::SndData() works.

Details: This was worked earlier by uncommenting "#define PL2303_COMPAT" from cdcprolific.h
I just re-added code by Oleg Mazurov which for some reason was lost during a later merge/pull request.
Also see: http://www.circuitsathome.com/mcu/major-acmprolific-bug-fix-posted-on-github