USB: document ioctl USBDEVFS_GET_SPEED

Documenting this ioctl along with the other ioctls

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20230724092843.18410-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>