[PATCH] spidernet: performance optimizations

Performance optimizations, changes in these areas:
  - RX and TX checksum offload
  - correct maximum MTU
  - don't use TX interrupts anymore, use a timer instead
  - remove some superfluous barriers
  - improve RX RAM full handling

From: Utz Bacher <utz.bacher@de.ibm.com>
Signed-off-by: Jens Osterkamp <jens.osterkamp@de.ibm.com>
Signed-off-by: Arnd Bergmann <arndb@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>