Change USB strings handling code

This commit add an extra field to the _usbd_device, that allows to
keep track of the number of USB strings  which allows simplify
boundaries checking code in usb_standard_get_descriptor.

This commit also changes the index base for strings in
usb_standard_get_descriptor which allows to get rid of necessity to
have a dummy one-character string in a strings array.

include/libopencm3/usb/usbd.h

@@ -38,7 +38,7 @@ extern u8 usbd_control_buffer[];
 extern int usbd_init(const usbd_driver *driver,
 		     const struct usb_device_descriptor *dev,
                      const struct usb_config_descriptor *conf,
-		     const char **strings);
+		     const char **strings, int num_strings);
 extern void usbd_set_control_buffer_size(u16 size);
 
 extern void usbd_register_reset_callback(void (*callback)(void));