net/mlx4_en: Fixes for DCBX

This patch adds a capability check before enabling DCBX.
In addition, it re-organizes the relevant data structures,
and fixes a typo in a define.

Fixes: af7d5185 ("net/mlx4_en: Add DCB PFC support through CEE netlink commands")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>