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

This is the fix for the Issue #109 I posted.

This used to work earlier by uncommenting "#define PL2303_COMPAT" from cdcprolific.h

I just re-added code from one of Oleg Mazurov's old commits (see commit: 3d068a063795505a6b577e31e2955577505f3d4e ) which for some reason was lost during a later merge/pull request (See commit: 3d068a063795505a6b577e31e2955577505f3d4e)