usb gadget serial: split out generic serial function

Split out the generic serial support into a "function driver".  This
closely mimics the ACM support, but with a MUCH simpler control model.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/gadget/u_serial.h b/drivers/usb/gadget/u_serial.h
index 83b45a3..f9ce300 100644
--- a/drivers/usb/gadget/u_serial.h
+++ b/drivers/usb/gadget/u_serial.h
@@ -58,5 +58,6 @@
 
 /* functions are bound to configurations by a config or gadget driver */
 int acm_bind_config(struct usb_configuration *c, u8 port_num);
+int gser_bind_config(struct usb_configuration *c, u8 port_num);
 
 #endif /* __U_SERIAL_H */