gve: Enable Link Speed Reporting in the driver.

This change allows the driver to report the device link speed
when the ethtool command:
	ethtool <nic name>
is run.
Getting the link speed is done via a new admin queue command:
ReportLinkSpeed.

Reviewed-by: Yangchun Fu <yangchun@google.com>
Signed-off-by: David Awogbemila <awogbemila@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>