igb: Add comment

Add explanatory comment to avoid confusion when a pointer is set
to the second word of an array instead of the customary cast of a
pointer to the beginning of the array.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>