net: ethernet: dec: tulip: Fix length mask in receive length calculation

The receive frame length calculation uses a wrong mask to calculate the
length of the received frames.

Per spec table 4-1 the length is contained in the FL (Frame Length)
field in bits 30:16.

This didn't show up as an issue so far since frames were limited to
1500 bytes which falls within the 11 bit window.
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/dec/tulip/de2104x.c

@@ -417,7 +417,10 @@ static void de_rx (struct de_private *de)
 		if (status & DescOwn)
 			break;
 
-		len = ((status >> 16) & 0x7ff) - 4;
+		/* the length is actually a 15 bit value here according
+		 * to Table 4-1 in the DE2104x spec so mask is 0x7fff
+		 */
+		len = ((status >> 16) & 0x7fff) - 4;
 		mapping = de->rx_skb[rx_tail].mapping;
 
 		if (unlikely(drop)) {