tg3: Fix RSS indirection table distribution

The current RSS indirection table is populated such that more traffic
will hit the first RSS ring.  This patch adjusts the indirection table
so that the load is more evenly distributed.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>