net/mlx5e: Fix multicast counter not up-to-date in "ip -s"

Currently the FW does not generate events for counters other than error
counters. Unlike ".get_ethtool_stats", ".ndo_get_stats64" (which ip -s
uses) might run in atomic context, while the FW interface is non atomic.
Thus, 'ip' is not allowed to issue FW commands, so it will only display
cached counters in the driver.

Add a SW counter (mcast_packets) in the driver to count rx multicast
packets. The counter also counts broadcast packets, as we consider it a
special case of multicast.
Use the counter value when calling "ip -s"/"ifconfig".

Fixes: f62b8bb8

 ("net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality")
Signed-off-by: Ron Diskin <rondi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>