net/mlx5e: Add link down events counter

Expose link_down_events counter through ethtool -S.
This counter is read from PPort statistics, then proccessed and stored as
a special handling software counter.
This counter is stored along software counters since it is the only PPort
counter that it's size is not 64 bits.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>