sky2: tx ring index mask fix

Mask for transmit ring status was picking up bits from the
unused sync ring.  They were always zero, so far...
Also, make sure to remind self not to make tx ring too big.
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>

drivers/net/sky2.c

@@ -1927,7 +1927,8 @@ static int sky2_status_intr(struct sky2_hw *hw, int to_do)
 
 		case OP_TXINDEXLE:
 			/* TX index reports status for both ports */
-			sky2_tx_done(hw->dev[0], status & 0xffff);
+			BUILD_BUG_ON(TX_RING_SIZE > 0x1000);
+			sky2_tx_done(hw->dev[0], status & 0xfff);
 			if (hw->dev[1])
 				sky2_tx_done(hw->dev[1],
 				     ((status >> 24) & 0xff)