usb: dwc3: switch trb enqueue/dequeue and first_trb_index to u8

We *know* that we have 1 PAGE (4096 bytes) for our
TRB poll. We also know the size of each TRB and know
that we can fit 256 of them in one PAGE. By using a
u8 type we can make sure that:

	enqueue++ % 256;

gets optimized to an increment only.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/core.h

@@ -470,8 +470,6 @@ struct dwc3_ep {
 
 	struct dwc3_trb		*trb_pool;
 	dma_addr_t		trb_pool_dma;
-	u32			trb_enqueue;
-	u32			trb_dequeue;
 	const struct usb_ss_ep_comp_descriptor *comp_desc;
 	struct dwc3		*dwc;
 
@@ -487,6 +485,18 @@ struct dwc3_ep {
 	/* This last one is specific to EP0 */
 #define DWC3_EP0_DIR_IN		(1 << 31)
 
+	/*
+	 * IMPORTANT: we *know* we have 256 TRBs in our @trb_pool, so we will
+	 * use a u8 type here. If anybody decides to increase number of TRBs to
+	 * anything larger than 256 - I can't see why people would want to do
+	 * this though - then this type needs to be changed.
+	 *
+	 * By using u8 types we ensure that our % operator when incrementing
+	 * enqueue and dequeue get optimized away by the compiler.
+	 */
+	u8			trb_enqueue;
+	u8			trb_dequeue;
+
 	u8			number;
 	u8			type;
 	u8			resource_index;
@@ -641,8 +651,8 @@ struct dwc3_request {
 	struct usb_request	request;
 	struct list_head	list;
 	struct dwc3_ep		*dep;
-	u32			first_trb_index;
 
+	u8			first_trb_index;
 	u8			epnum;
 	struct dwc3_trb		*trb;
 	dma_addr_t		trb_dma;