ixgbe: add link_down_events statistic

ixgbe: add link_down_events statistic

Introduce a link_down_events counter to the ixgbe 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>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Martyna Szapar-Mudlaw <martyna.szapar-mudlaw@linux.intel.com>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>