ice: add link_down_events statistic

ice: add link_down_events statistic

Introduce a link_down_events counter to the ice 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.

Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Martyna Szapar-Mudlaw <martyna.szapar-mudlaw@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>