usb: gadget: f_dfu: Add get_alt function to pass the USB compliance test

Without this function the USB compliance test (USB 2.0 Command Verifier) will
fail in the "Interface Descriptor Test" with this error message:

FAIL
(1.2.51) A successful GetInterface request must return the alternate setting
set by a prior call to SetInterface.

Lets add this function to read back the value so that the DFU device fully
passes the USB compliance test.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Samuel Egli <samuel.egli@siemens.com>
Cc: Enrico Leto <enrico.leto@siemens.com>
Acked-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>

drivers/usb/gadget/f_dfu.c

@@ -780,6 +780,13 @@ static int dfu_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 	return 0;
 }
 
+static int __dfu_get_alt(struct usb_function *f, unsigned intf)
+{
+	struct f_dfu *f_dfu = func_to_dfu(f);
+
+	return f_dfu->altsetting;
+}
+
 /* TODO: is this really what we need here? */
 static void dfu_disable(struct usb_function *f)
 {
@@ -806,6 +813,7 @@ static int dfu_bind_config(struct usb_configuration *c)
 	f_dfu->usb_function.bind = dfu_bind;
 	f_dfu->usb_function.unbind = dfu_unbind;
 	f_dfu->usb_function.set_alt = dfu_set_alt;
+	f_dfu->usb_function.get_alt = __dfu_get_alt;
 	f_dfu->usb_function.disable = dfu_disable;
 	f_dfu->usb_function.strings = dfu_generic_strings;
 	f_dfu->usb_function.setup = dfu_handle;