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>
4 jobs for fix-USB-BusID-parsing in 6 minutes and 2 seconds (queued for 2 seconds)
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Status Job ID Name Coverage
  Docker Image
passed #5794568
debian-buster

00:00:13

 
  Build And Test
passed #5794569
autotools

00:05:49

passed #5794570
meson

00:02:06

passed #5794571
meson-noglamor

00:01:53