linux: Assume usbfs path = /dev/bus/usb when using UDEV

linux: Assume usbfs path = /dev/bus/usb when using UDEV

On some exotic hardware, e.g. HP ProLiant Moonshot Cartridge servers there
are no usb controllers, so no usb devices at all.

In this case currently libusb_init will fail, because find_usbfs_path
fails. Many apps don't handle this gracefully, and even if they do not crash
the result still is not pretty, e.g.:

unable to initialize libusb: -99

Where one simply would expect empty output.

Since on systems using udev the usbfs path should always be /dev/bus/usb
(as that gets created by udev), simply assume /dev/bus/usb when build with
USE_UDEV and the path cannot be found in the traditional way.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>