Mount setup - don't look at non-usb devices

[wtgee]

The pocs mount setup script will loop through everything found by pyserial, but we should do some filtering to only ports that it might be on. Currently if you try to set up the mount after the power is running it will try to probe the port the power is runnig on (e.g. on /dev/ttyACM0), which will screw things up.