sky2: Avoid races in sky2_down

Reset rx chain before trying to drain it.
Shut interrupts off last, incase there's something to report.

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