staging/fwserial: Update TODO file per reviewer comments

Pursuant to this review https://lkml.org/lkml/2012/11/12/500
by Stefan Richter, update the TODO file.
- Clarify purpose of TODO file
- Remove firewire item #4. As discussed in this conversation
  https://lkml.org/lkml/2012/11/13/564

 knowing the AR buffer size
  is not a hard requirement. The required rx buffer size can be
  determined experimentally.
- Remove firewire item #5. This was a private note for further
  experimentation.
- Change firewire item #1. Change suggested header from uapi header
  to kernel-only header.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>