usb: gadget: f_uac2: fix bFirstInterface in composite gadget

If there are multiple functions associated with a configuration, then
the UAC2 interfaces may not start at zero.  Set the correct first
interface number in the association descriptor so that the audio
interfaces are enumerated correctly in this case.
Reviewed-by: NKrzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: NJohn Keeping <john@metanate.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_uac2.c

@@ -524,6 +524,8 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 		dev_err(dev, "%s:%d Error!\n", __func__, __LINE__);
 		return ret;
 	}
+	iad_desc.bFirstInterface = ret;
+
 	std_ac_if_desc.bInterfaceNumber = ret;
 	uac2->ac_intf = ret;
 	uac2->ac_alt = 0;