Increase static number of nframes which enables very initial ISOC support,

Increase static number of nframes which enables very initial ISOC support,
e.g. uvideo(4) with lower resolutions.

In general we might need to re-write parts of the nframes handling in the
driver, since the NetBSD nframes transfer allocation doesn't match with our
USB stack.

With this we can at least start further testing and improvement for ISOC
support.