fbnic: Add handler for reporting link down event statistics

fbnic: Add handler for reporting link down event statistics

We were previously not displaying the number of link_down_events tracked by
the device. With this change we should now be able to display the value.
The value itself tracks the calls from the phylink interface to the
mac_link_down call.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/176374323824.959489.6915296616773178954.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Paolo Abeni <pabeni@redhat.com>