igb: correctly offset 82575 flow control watermarks by 16 bytes

The watermark values for the 82575 were not being set correctly.  As a
result the high and low watermark values were set to the same value which
can lead to excess xon/xoff packets being generated.

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>