[TG3]: Handle tg3_init_rings() failures

Handle dev_alloc_skb() failures when initializing the RX rings.
Without proper handling, the driver will crash when using a partial
ring.

Thanks to Stephane Doyon <sdoyon@max-t.com> for reporting the bug and
providing the initial patch.

Howie Xu <howie@vmware.com> also reported the same issue.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>