Fix USBDMXPro buffer length for Enttec

For ENTTEC USB pro, the data length must include start code lenth. Add 1
to the length in USBDMXPro.c
Include "serialutil.h"
Merge pull request #56 from ihbar/ihbar_0

Fix USBDMXPro buffer length for Enttec.

cpinkham note: The +1 was left off when setting this up originally, this should fix any device using the DMX Pro spec.