sky2: fix transmit DMA map leakage

The book keeping structure for transmit always had the flags value
cleared so transmit DMA maps were never released correctly.
Based on patch by Jarek Poplawski, problem observed by Michael Breuer.

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