net/mlx5e: Add discard counters per priority

Add counters that count (per priority) the number of received
packets that dropped due to lack of buffers on a physical port. If
this counter is increasing, it implies that the adapter is
congested and cannot absorb the traffic coming from the network.

Signed-off-by: Aharon Landau <aharonl@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>