sky2: Reset tx train after interrupts disabled.

Reseting the tx chain too soon results in invalid tx queue positions
being delivered in the status queue.  This also makes sure there's no
overlap between the cleanup done by sky2_tx_clean() and
sky2_tx_done().

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>