e1000e: use hardware writeback batching

Most e1000e parts support batching writebacks.  The problem with this is
that when some of the TADV or TIDV timers are not set, Tx can sit forever.

This is solved in this patch with write flushes using the Flush Partial
Descriptors (FPD) bit in TIDV and RDTR.

This improves bus utilization and removes partial writes on e1000e,
particularly from 82571 parts in S5500 chipset based machines.

Only ES2LAN and 82571/2 parts are included in this optimization, to reduce
testing load.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>