sky2: EC-U performance and jumbo support

The Yukon EC Ultra chips have transmit settings for store and
forward and PCI buffering. By setting these appropriately, normal
performance goes from 750Mbytes/sec to 940Mbytes/sec (non-jumbo).

It is also possible to do Jumbo mode, but it means turning off
TSO and checksum offload so the performance gets worse. There isn't
enough buffering for checksum offload to work.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>