net: dummy: use standard dev_lstats_add() and dev_lstats_read()

This driver can simply use the common infrastructure instead
of duplicating it.

This cleanup will ease u64_stats_t adoption in a single location.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>