sky2: Create buffer alloc and free helpers

Refactor similar two sections of code that free buffers into one.
Only call tx_init if all buffer allocations 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>