that code is generating the fallowing warning:

drivers/usb/gadget/serial.c:162: warning: `debug' defined but not used

 When G_SERIAL_DEBUG is not defined, `debug' is not used, because
the gs_debug() function is compiled only when G_SERIAL_DEBUG is
defined.

 Thus, a solution is to define `debug' only when G_SERIAL_DEBUG
is defined. That includes the use of `debug' as a module parameter
(this last part I'm not sure).

 The patch bellow does that (compiles ok, not tested because I don't
have that hardware):