usb: introduce usb_ep_type_string() function

In some places, the code prints a human-readable USB endpoint
transfer type (e.g. "bulk"). This involves a switch statement
sometimes wrapped around in ({ ... }) block leading to code
repetition.
To make this scenario easier, here introduces usb_ep_type_string()
function, which returns a human-readable name of provided
endpoint type.
It also changes a few places switch was used to use this
new function.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/udc/dummy_hcd.c

@@ -617,21 +617,7 @@ static int dummy_enable(struct usb_ep *_ep,
 		_ep->name,
 		desc->bEndpointAddress & 0x0f,
 		(desc->bEndpointAddress & USB_DIR_IN) ? "in" : "out",
-		({ char *val;
-		 switch (usb_endpoint_type(desc)) {
-		 case USB_ENDPOINT_XFER_BULK:
-			 val = "bulk";
-			 break;
-		 case USB_ENDPOINT_XFER_ISOC:
-			 val = "iso";
-			 break;
-		 case USB_ENDPOINT_XFER_INT:
-			 val = "intr";
-			 break;
-		 default:
-			 val = "ctrl";
-			 break;
-		 } val; }),
+		usb_ep_type_string(usb_endpoint_type(desc)),
 		max, ep->stream_en ? "enabled" : "disabled");
 
 	/* at this point real hardware should be NAKing transfers