USB: usbtest - ensure correct isoc data length

Check the data length of isochronous transfers is
as expected.

With this test #16 will now fail if the device side
sends no data.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>