igb: use packet buffer sizes from RXPBS register

This patch changes the configuration for 82576 so that it uses the actual
value of the 82576 rx packet buffer size instead of just assuming the
value.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>