e1000: Jumbo frames fixes for 82573

Disable jumbo frames for 82573L alltogether and when ASPM is enabled
since the hardware has problems with it. For the NICs that do support
this in the 82573 series we set ERT_2048 to attempt to receive as much
traffic as early as we can.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>