libdrm: Handle usb_interface devices for usb parsing
Currently the code expects that the device found at
/sys/char/maj:
min/device for USB devices is a "usb_device". However,
at least for some devices, such as for the udl driver, they are instead
a "usb_interface".
A usb_interface is a child of the usb_device we're interested in, so we walk up one in the /sys path to get there.
Signed-off-by: Scott Anderson scott@anderso.nz