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>