ibmvnic: Halt TX and report carrier off on H_CLOSED return code

This patch disables transmissions and reports carrier off if xmit
function returns that the hardware TX queue is closed. The driver can
then await a signal from firmware to determine the correct reset method.

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