USB: usbtest: support test device with only one iso-in or iso-out endpoint

It is very common that one altsetting may include only one iso-in or iso-out
single endpoint, especially for high bandwidth endpoint, so support it.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>