usb: class: Use USB_CLASS_PRINTER instead of number 7

Kernel provides very nice defines for USB device class
so it's a good idea to use them in suitable places.
It is much easier to grep for such define instead of 7.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>