Do not configure ports if not necessary

Some USB CDC devices (such as PX4 autopilots on USB) do not support any of the port config options, as USB doesn't know about flow control or baud rate. This is a workaround for these calls making NuttX USB device unhappy. Tested with a 3DR radio / FTDI for other ports, does have no effects on those.

Commented out the DTR reset stuff for APM - is this any good / needed? @DonLakeFlyer Could you pitch in here?