usb: gadget: u_serial: convert into a module

Every user of u_serial has now to select the U_SERIAL symbol instead of
including the file.
There is one limition with this: ports and and gs_tty_driver are global
variables in u_serial. Since all users share them, there can be only one
user loaded at a time i.e. either g_serial or g_nokia.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/multi.c

@@ -42,7 +42,6 @@ MODULE_LICENSE("GPL");
  */
 #include "f_mass_storage.c"
 
-#include "u_serial.c"
 #include "f_acm.c"
 
 #include "f_ecm.c"