usb: dwc3: gadget: rename busy/free_slot to trb_enqueue/dequeue

This makes it clear that we're dealing with a queue
of TRBs. No functional changes. While at that, also
rename start_slot to first_trb_index for similar
reasons.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
index 5e0a21b..143deb4 100644
--- a/drivers/usb/dwc3/ep0.c
+++ b/drivers/usb/dwc3/ep0.c
@@ -70,10 +70,10 @@
 		return 0;
 	}
 
-	trb = &dwc->ep0_trb[dep->free_slot];
+	trb = &dwc->ep0_trb[dep->trb_enqueue];
 
 	if (chain)
-		dep->free_slot++;
+		dep->trb_enqueue++;
 
 	trb->bpl = lower_32_bits(buf_dma);
 	trb->bph = upper_32_bits(buf_dma);
@@ -845,7 +845,7 @@
 			trb++;
 			length = trb->size & DWC3_TRB_SIZE_MASK;
 
-			ep0->free_slot = 0;
+			ep0->trb_enqueue = 0;
 		}
 
 		transfer_size = roundup((ur->length - transfer_size),