usb: Provide usb_speed_string() function

In a few places in the kernel, the code prints
a human-readable USB device speed (eg. "high speed").
This involves a switch statement sometimes wrapped
around in ({ ... }) block leading to code repetition.

To mitigate this issue, this commit introduces
usb_speed_string() function, which returns
a human-readable name of provided speed.

It also changes a few places switch was used to use
this new function.  This changes a bit the way the
speed is printed in few instances at the same time
standardising it.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/usb/gadget/net2272.c b/drivers/usb/gadget/net2272.c
index 6fef1c0..08a4a36 100644
--- a/drivers/usb/gadget/net2272.c
+++ b/drivers/usb/gadget/net2272.c
@@ -1764,8 +1764,8 @@
 				dev->gadget.speed = USB_SPEED_HIGH;
 			else
 				dev->gadget.speed = USB_SPEED_FULL;
-			dev_dbg(dev->dev, "%s speed\n",
-				(dev->gadget.speed == USB_SPEED_HIGH) ? "high" : "full");
+			dev_dbg(dev->dev, "%s\n",
+				usb_speed_string(dev->gadget.speed));
 		}
 
 		ep = &dev->ep[0];