Introduce and use BUS_USB

With !155, the device bus ID received via udev is constructed
properly with the "usb:" prefix. But, it is not enough to
make the following line to work in Section "Device":

    BusID  "usb:0:1.2:1.0"

Introduce BUS_USB, so the prefix can be distinguished from BUS_PCI
and check the supplied BusID value against device->attribs->busid
in xf86PlatformDeviceCheckBusID().

Signed-off-by: Böszörményi Zoltán <zboszor@pr.hu>
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