USB: drivers/usb/serial/bus.c: Fix incompatible pointer type warning

drivers/usb/serial/bus.c: In function usb_serial_bus_deregister:
drivers/usb/serial/bus.c:185:
warning: passing argument 1 of free_dynids from incompatible pointer type

Above build warning comes when CONFIG_HOTPLUG=n because argument of
free_dynids() in serial/bus.c is a struct usb_serial_driver, not a
struct usb_driver. This is not a runtime bug, because the function
is an empty stub and never dereferences the passed pointer anyway.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>