Raising UVIDEO_NFRAMES_MAX from 40 to 128 in 3b6f833c95eb improved
throughput on xhci but made every camera on an ehci bus fail to
stream. Integrated webcams became unusable.
Measured on a MacBookPro9,2 with two ehci(4) FaceTime HD cameras and an
xhci(4) Logitech C920:
128 32 ehci, 12 captures 0 ok 12 ok xhci 1920x1080 5 fps 5 fps xhci 1280x720 10 fps 10 fps
Fixes: 3b6f833c95eb