ionic: add support for ethtool extended stat link_down_count

Following the example of 'commit 9a0f830f ("ethtool: linkstate:
add a statistic for PHY down events")', added support for link down
events.

Add callback ionic_get_link_ext_stats to ionic_ethtool.c to support
link_down_count, a property of netdev that gets reported exclusively
on physical link down events.

Run ethtool -I <devname> to display the device link down count.

Signed-off-by: Nitya Sunkad <nitya.sunkad@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>