net/mlx5: fix route Netlink message overflow

net/mlx5: fix route Netlink message overflow

Route Netlink message socket is wrongly initialized by registering to
the route link group.  This causes the socket to receive all link
message related to routes whereas the PMD do not expect to receive such
information.  In some situation it ends by filling the socket at a point
that any new message cannot be exchanged.
As the PMD is not expected to process such broadcast messages, the
parameter in the nl_group in the function is also remove.

Fixes: ccdcba53a3f4 ("net/mlx5: use Netlink to add/remove MAC addresses")
Cc: stable@dpdk.org
Signed-off-by: Zijie Pan <zijie.pan@6wind.com>
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>