mlx4: remove custom dev_addr clearing

mlx4_en_u64_to_mac() takes the dev->dev_addr pointer and writes
to it byte by byte. It also clears the two bytes _after_ ETH_ALEN
which seems unnecessary. dev->addr_len is set to ETH_ALEN just
before the call.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>