uvideo: increase isochronous transfer depth for throughput

Increase NFRAMES_MAX from 40 to 128 and IXFERS from 3 to 5 to keep
more packets in flight on the USB bus. This brings throughput from
~13.5 MB/s to ~21 MB/s (for comparison on the same camera webcamd
provided ~20MB/s.

The linux driver also uses 5 IXFERS (but only 32 NFRAMES_MAX)

Tested by: manu