V4L/DVB (5523): Usbvision: fix a debug message in usb probe function

- change a printk to PDEBUG when USB probe detects an unknown device.
This will avoid a message log from usbvision when an unclaimed device is inserted.

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>