usb: common: introduce usb_state_string()

this function will receive enum usb_device_state
and return a human-readable string from it or,
case an unknown value is passed as argument,
the string "UNKNOWN".

Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/ch9.h

@@ -43,4 +43,13 @@
  */
 extern const char *usb_speed_string(enum usb_device_speed speed);
 
+
+/**
+ * usb_state_string - Returns human readable name for the state.
+ * @state: The state to return a human-readable name for. If it's not
+ *	any of the states devices in usb_device_state_string enum,
+ *	the string UNKNOWN will be returned.
+ */
+extern const char *usb_state_string(enum usb_device_state state);
+
 #endif /* __LINUX_USB_CH9_H */