sky2: transmit ring 64 bit conservation

This patch saves elements on transmit ring by only updating the upper
64 bit address when it changes. With many workloads skb's are located
in same region, so it saves space.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>