gve: Use link status register to report link status

This makes the driver better aware of the connectivity status of the
device. Based on the device's status register, the driver can call
netif_carrier_{on,off}.

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