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>
diff --git a/drivers/usb/gadget/dbgp.c b/drivers/usb/gadget/dbgp.c
index 87d1650..41eb98d 100644
--- a/drivers/usb/gadget/dbgp.c
+++ b/drivers/usb/gadget/dbgp.c
@@ -13,9 +13,7 @@
 #include <linux/usb/ch9.h>
 #include <linux/usb/gadget.h>
 
-#ifdef CONFIG_USB_G_DBGP_SERIAL
-#include "u_serial.c"
-#endif
+#include "u_serial.h"
 
 #define DRIVER_VENDOR_ID	0x0525 /* NetChip */
 #define DRIVER_PRODUCT_ID	0xc0de /* undefined */