tools: usb: testusb: update default vary for superspeed

Currently, default vary will not accomodate superspeed endpoints
causing unexpected babble errors in the IN direction. Let's update
default 'vary' parameter so that we can maintain a "short-less"
transfer as hinted at the comment.

Reported-by: Ammy Yi <ammy.yi@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>