cdc: Fix build warning.

   In file included from drivers/usb/gadget/function/u_serial.h:16:0,
                    from drivers/usb/gadget/function/f_acm.c:23:
>> include/linux/usb/cdc.h:47:5: warning: 'struct usb_interface' declared inside parameter list
        int buflen);
        ^
>> include/linux/usb/cdc.h:47:5: warning: its scope is only this definition or declaration, which is probably not what you want

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>