[media] tm6000: Properly count device usage

When the USB device is disconnected, the device usage bit is not cleared
properly. This leads to errors when a device is unplugged and replugged
several times until all TM6000_MAXBOARDS bits are used and keeps the
driver from binding to the device.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>