Setup an error when QSP::setBaudRate() fails on Unix

Inside of a method is missed setup of an error code, therefore always
returns QSP::UnknownError (or some previous error).

Tested on Linux with USB PL2303, using Terminal example (when trying to
setup of an unsupported 14400 baud rate).

Change-Id: I6b3143dd6d451a32024a5ca5239dac826ceb7af6
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>