net/ibmvnic: Update carrier state after link state change

Only set the device carrier state to on after receiving an up link
state indication from the underlying adapter. Likewise, if a down
link indication is receieved, update the carrier state accordingly.
This fix ensures that accurate carrier state is reported by the driver
following a link state update by the underlying adapter.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>