[TG3]: Fix possible NULL deref in tg3_run_loopback().

tg3_run_loopback doesn't check that dev_alloc_skb() returns anything
useful.

Even if dev_alloc_skb() fails to return an skb to us we'll happily go
on and assume it did, so we risk dereferencing a NULL pointer.  Much
better to fail gracefully by returning -ENOMEM than crashing here.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>