sky2: Allocate initial skbs in sky2_alloc_buffers

Allocate everything in one place so there's a single point
of failure in sky2_up, and sky2_rx_start can no longer fail.

Don't leave the hardware in a partially initialized state in the
case rx ring allocation fails.

As with the old code, the rx ring still needs to be fully
allocated for sky2_up to succeed.

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