ixgbe: add comment on SFP+ ID for Active DA

These comments were forgotten in the initial patch to add this
functionality.  This patch corrects that.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>