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>

include/linux/usb/ch9.h

@@ -868,6 +868,18 @@ enum usb_device_speed {
 	USB_SPEED_SUPER,			/* usb 3.0 */
 };
 
+#ifdef __KERNEL__
+
+/**
+ * usb_speed_string() - Returns human readable-name of the speed.
+ * @speed: The speed to return human-readable name for.  If it's not
+ *   any of the speeds defined in usb_device_speed enum, string for
+ *   USB_SPEED_UNKNOWN will be returned.
+ */
+extern const char *usb_speed_string(enum usb_device_speed speed);
+
+#endif
+
 enum usb_device_state {
 	/* NOTATTACHED isn't in the USB spec, and this state acts
 	 * the same as ATTACHED ... but it's clearer this way.