sky2: lock less transmit completion

Transmit completion can safely run lockless against transmit start.
In the normal case, completion is done from NAPI and only looks
at elements that are at the tail of the ring.  When doing shutdown
or reset, the transmiter should be completely block by NAPI disable
and blocking of transmit queue. 

Based on earlier work by Mike McCormack.

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