uvideo: limit isochronous transfers to 32 frames

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

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D58501