igb: add defaults for i210 TX/RX PBSIZE

Set the defaults on probe for the packet buffer size registers for the
i210.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>