i40e: add link_down_events statistic

i40e: add link_down_events statistic

Introduce a link_down_events counter to the i40e driver, incremented
each time the link transitions from up to down.
This counter can help diagnose issues related to link stability,
such as port flapping or unexpected link drops.

The value is exposed via ethtool's get_link_ext_stats() interface.

Co-developed-by: Martyna Szapar-Mudlaw <martyna.szapar-mudlaw@linux.intel.com>
Signed-off-by: Martyna Szapar-Mudlaw <martyna.szapar-mudlaw@linux.intel.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>