usb: dwc3: ep0: use gadget->isoch_delay for isoch_delay value

Instead of keeping our own isoch_delay, let's make use of the newly
introduced isoch_delay member in struct usb_gadget. The benefit here
is that we would be using a generic "API" which other UDCs can use,
resulting in a common setup for gadget drivers who may be interested
in Isoch Delay value.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/core.h

@@ -796,7 +796,6 @@ struct dwc3_scratchpad_array {
  * @usb2_generic_phy: pointer to USB2 PHY
  * @usb3_generic_phy: pointer to USB3 PHY
  * @ulpi: pointer to ulpi interface
- * @isoch_delay: wValue from Set Isochronous Delay request;
  * @u2sel: parameter from Set SEL request.
  * @u2pel: parameter from Set SEL request.
  * @u1sel: parameter from Set SEL request.
@@ -955,7 +954,6 @@ struct dwc3 {
 	enum dwc3_ep0_state	ep0state;
 	enum dwc3_link_state	link_state;
 
-	u16			isoch_delay;
 	u16			u2sel;
 	u16			u2pel;
 	u8			u1sel;

drivers/usb/dwc3/ep0.c

@@ -736,11 +736,7 @@ static int dwc3_ep0_set_isoch_delay(struct dwc3 *dwc, struct usb_ctrlrequest *ct
 	if (wIndex || wLength)
 		return -EINVAL;
 
-	/*
-	 * REVISIT It's unclear from Databook what to do with this
-	 * value. For now, just cache it.
-	 */
-	dwc->isoch_delay = wValue;
+	dwc->gadget.isoch_delay = wValue;
 
 	return 0;
 }