fiq_fsm: turbocharge high-speed isochronous

HS Isoc basically means "do this thing lots of times to this
endpoint" where "thing" is either request data or send data.
We can accelerate this in the FIQ by handing over the
iso_frame_descs struct and pointing the FIQ at the transfer
buffer.

There is a large tradeoff however: for each isochronous HS
endpoint active in the FIQ, a host channel will be eaten.