sky2: Refactor down/up code out of sky2_restart()

Code to bring down all sky2 interfaces and bring it up
again can be reused in sky2_suspend and sky2_resume.

Factor the code to bring the interfaces down into
sky2_all_down and the up code into sky2_all_up.

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