Previously we wrote 4 x 64 byte transactions to the downstream device.
This is fast, but very occasionally the USB host silicon would bug out.
Reverting to single 64 byte write transactions increases reliability at
the expense of some write speed.
some flash drives can take up to 2 seconds to write a single block.
Also, reverting the previous two commits because they didn't actually
help. Aughhhhhh...
Robert Fisk