USB: usb-serial: fix a sparse warning about different signedness

fix the following sparse warning:

drivers/usb/serial/usb-serial.c:927:43: warning: incorrect type in argument 3 (different signedness)
drivers/usb/serial/usb-serial.c:927:43:    expected unsigned int *minor
drivers/usb/serial/usb-serial.c:927:43:    got int *<noident>
  CHECK   drivers/usb/serial/generic.c

Signed-off-by: Andre Haupt <andre@bitwigglers.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>