USB: serial: bus: fix const issue in usb_serial_device_match()

usb_serial_device_match() takes a const pointer, and then decides to
cast it away into a non-const one, which is not a good thing to do
overall.  Fix this up by properly setting the pointers to be const to
preserve that attribute.

Fixes: d69d8048 ("driver core: have match() callback in struct bus_type take a const *")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>