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: NAlexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/igb/igb_main.c

@@ -1306,13 +1306,8 @@ void igb_reset(struct igb_adapter *adapter)
 	hwm = min(((pba << 10) * 9 / 10),
 			((pba << 10) - 2 * adapter->max_frame_size));
 
-	if (mac->type < e1000_82576) {
-		fc->high_water = hwm & 0xFFF8;	/* 8-byte granularity */
-		fc->low_water = fc->high_water - 8;
-	} else {
-		fc->high_water = hwm & 0xFFF0;	/* 16-byte granularity */
-		fc->low_water = fc->high_water - 16;
-	}
+	fc->high_water = hwm & 0xFFF0;	/* 16-byte granularity */
+	fc->low_water = fc->high_water - 16;
 	fc->pause_time = 0xFFFF;
 	fc->send_xon = 1;
 	fc->current_mode = fc->requested_mode;