USB: serial: metro-usb: dummy implement write_int_callback() function.

This function is never called now. Because we don`t send much data
to the device, only one byte via usb_interrupt_msg(). That doesn't require
callback function. But without declaration of write_int_callback inside
the struct usb_serial_driver, the usb_serial_probe doesn't initialize
endpoint address for the interrupt out pipe(interrupt_out_endpointAddress).
This endpoint is necessary for sending data via usb_interrupt_msg()
function.

Signed-off-by: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>