usb: gadget: f_serial: Ensure gserial disconnected during unbind

Some UDCs may return an error during pullup disable as part of the
unbind path for a USB configuration.  This will lead to a scenario
where the disable() callback is skipped, whereas the unbind() still
occurs.  If this happens, the u_serial driver will continue to fail
subsequent binds, due to an already existing entry in the ports array.
Ensure that gserial_disconnect() is called during the f_serial unbind,
so the ports entry is properly cleared.
Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Link: https://lore.kernel.org/r/20220111064850.24311-1-quic_wcheng@quicinc.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_serial.c

@@ -345,6 +345,10 @@ static void gser_free(struct usb_function *f)
 
 static void gser_unbind(struct usb_configuration *c, struct usb_function *f)
 {
+	struct f_gser	*gser = func_to_gser(f);
+
+	/* Ensure port is disconnected before unbinding */
+	gserial_disconnect(&gser->port);
 	usb_free_all_descriptors(f);
 }