macvlan: allow to enqueue broadcast pkt on virtual device

Since commit 412ca155 ("macvlan: Move broadcasts into a work queue"), the
driver uses tx_queue_len of the master device as the limit of packets enqueuing.
Problem is that virtual drivers have this value set to 0, thus all broadcast
packets were rejected.
Because tx_queue_len was arbitrarily chosen, I replace it with a static limit
of 1000 (also arbitrarily chosen).

CC: Herbert Xu <herbert@gondor.apana.org.au>
Reported-by: Thibaut Collet <thibaut.collet@6wind.com>
Suggested-by: Thibaut Collet <thibaut.collet@6wind.com>
Tested-by: Thibaut Collet <thibaut.collet@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>