USB: g_printer: fix bug in unregistration

The allocated chardevice region range is only 1 device but on
unregister it currently tries to deregister 2.

Found this while doing a insmod/rmmod/insmod/rm... of the module
which seemed to eat major numbers.

Signed-off-by: Fabian Godehardt <fg@emlix.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>