net/mlx5e: Expose PFC stall prevention counters

net/mlx5e: Expose PFC stall prevention counters

Add the needed capability bit and counters to device spec description.
Expose the following two counters in ethtool:

tx_pause_storm_warning_events: when the device is stalled for a period
longer than a pre-configured watermark, the counter increase, allowing
the debug utility an insight into current device status.

tx_pause_storm_error_events: when the device is stalled for a period
longer than a pre-configured timeout, the pause transmission is disabled,
and the counter increase.

Signed-off-by: Inbar Karmy <inbark@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>