net/mlx5e: Add interface down dropped packets statistics

Added the following packets drop counter:
Rx interface down dropped packets - counts packets which were received
while the ETH interface was down.
This counter will be shown on ethtool as a new counter called
rx_if_down_packets.

The implementation allocates a q_counter for drop rq which gets all the
received traffic while the interface is down.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>