e1000: look in the page and not in skb->data for the last byte

The code seems to want to look at the last byte where the HW puts some
information. Since the skb->data area is never seen by the HW I guess it
does not work as expected. We pass the page address to the HW so I
*think* in order to get to the last byte where the information might be
one should use the page buffer and take a look.
This is of course not more than just compile tested.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>