usb: dwc3: gadget: check link trb after free_slot is increased

In ISOC transfers, when free_slot points to the last TRB (i.e. Link
TRB), and all queued requests meet Missed Interval Isoc error, busy_slot
points to trb0.
	busy_slot->trb0
		   trb1
		   ...
	free_slot->trb31(Link TRB)

After end transfer and receiving the XferNotReady event, trb_left is
caculated as 1 which is wrong, and no TRB will be primed to the
endpoint.

The root cause is free_slot is not increased the same way as busy_slot.
When busy_slot is increased by one, it checks if points to a link TRB
after increasement, but free_slot checks it before increasement.
free_slot should behave the same as busy_slot to make the trb_left
caculation correct.
Reviewed-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Zhuang Jin Can <jin.can.zhuang@intel.com>
Signed-off-by: Jiebing Li <jiebing.li@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/gadget.c

@@ -828,10 +828,6 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 			length, last ? " last" : "",
 			chain ? " chain" : "");
 
-	/* Skip the LINK-TRB on ISOC */
-	if (((dep->free_slot & DWC3_TRB_MASK) == DWC3_TRB_NUM - 1) &&
-			usb_endpoint_xfer_isoc(dep->endpoint.desc))
-		dep->free_slot++;
 
 	trb = &dep->trb_pool[dep->free_slot & DWC3_TRB_MASK];
 
@@ -843,6 +839,10 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 	}
 
 	dep->free_slot++;
+	/* Skip the LINK-TRB on ISOC */
+	if (((dep->free_slot & DWC3_TRB_MASK) == DWC3_TRB_NUM - 1) &&
+			usb_endpoint_xfer_isoc(dep->endpoint.desc))
+		dep->free_slot++;
 
 	trb->size = DWC3_TRB_SIZE_LENGTH(length);
 	trb->bpl = lower_32_bits(dma);