inga_tool: Don't libusb_unref_device() the device we are going to use

If we're going to open an usb device, don't call
libusb_free_device_list() with unref_devices set to 1. Instead
manually call libusb_unref_device() on all other devices, and call it
again when freeing the usb context.
Merge pull request #3 from DrMcCoy/inga_tool_fixusbunref

inga_tool: Don't libusb_unref_device() the device we are going to use