e1000e: increase Rx PBA to prevent dropping received packets on 82566/82567

During bi-directional stress on some 82566/82567 devices, some received
packets were dropped.  Increasing the Receive Packet Buffer Allocation
resolves this.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>