spidernet: separate hardware state from driver state.

This patch separates the hardware descriptor state from the
driver descriptor state, per (old) suggestion from Ben Herrenschmidt.
This compiles and boots and seems to work.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: Jens Osterkamp <Jens.Osterkamp@de.ibm.com>
Cc: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Jeff Garzik <jeff@garzik.org>