mlx4_en: Fix loss of promiscuity

The mlx4_en driver uses the combination stop_port/start_port
in a number of places.  Unfortunately that causes any promiscuous
mode settings on the hardware to be lost.

This patch fixes that problem.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>