usb: gadget: function: f_obex: fix Interface Descriptor Test

On USB20CV's Interface Descriptor Test, a series
of SetInterface/GetInterface requests are issued
and gadget driver is required to always return
correct alternate setting.

In one step of the test, g_serial with f_obex
was returning the wrong value (1 instead of 0).

In order to fix this, we will now hold currently
selected alternate setting inside our struct f_obex
and just return that from our ->get_alt()
implementation.

Note that his also simplifies the code a bit.
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_obex.c

@@ -35,6 +35,7 @@ struct f_obex {
 	struct gserial			port;
 	u8				ctrl_id;
 	u8				data_id;
+	u8				cur_alt;
 	u8				port_num;
 	u8				can_activate;
 };
@@ -235,6 +236,8 @@ static int obex_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 	} else
 		goto fail;
 
+	obex->cur_alt = alt;
+
 	return 0;
 
 fail:
@@ -245,10 +248,7 @@ static int obex_get_alt(struct usb_function *f, unsigned intf)
 {
 	struct f_obex		*obex = func_to_obex(f);
 
-	if (intf == obex->ctrl_id)
-		return 0;
-
-	return obex->port.in->driver_data ? 1 : 0;
+	return obex->cur_alt;
 }
 
 static void obex_disable(struct usb_function *f)