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/file_storage.c b/drivers/usb/gadget/file_storage.c
index 39ece40..4ac8084 100644
--- a/drivers/usb/gadget/file_storage.c
+++ b/drivers/usb/gadget/file_storage.c
@@ -2862,17 +2862,10 @@
 		fsg->config = new_config;
 		if ((rc = do_set_interface(fsg, 0)) != 0)
 			fsg->config = 0;	// Reset on errors
-		else {
-			char *speed;
-
-			switch (fsg->gadget->speed) {
-			case USB_SPEED_LOW:	speed = "low";	break;
-			case USB_SPEED_FULL:	speed = "full";	break;
-			case USB_SPEED_HIGH:	speed = "high";	break;
-			default: 		speed = "?";	break;
-			}
-			INFO(fsg, "%s speed config #%d\n", speed, fsg->config);
-		}
+		else
+			INFO(fsg, "%s config #%d\n",
+			     usb_speed_string(fsg->gadget->speed),
+			     fsg->config);
 	}
 	return rc;
 }