NTB: ntb_netdev not covering all receive errors

ntb_netdev is allowing the link to come up even when -ENOMEM is returned
from ntb_transport_rx_enqueue.  Fix to cover all possible errors.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>