igb: Only sync size of expected frame in ethtool testing

We only need to sync the size of the frame that is read to test.  We don't
need to sync the entire Rx buffer.  This way the testing is more consistent
with how we handle things in the receive path.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>