ntb: Drop packets when qp link is down

Currently when the transport receive packets after netdev has closed the
transport returns error and triggers tx errors to be incremented and
carrier to be stopped. There is no reason to return error if the device is
already closed. Drop the packet and return 0.

Fixes: e26a5843

 ("NTB: Split ntb_hw_intel and ntb_transport drivers")
Reported-by: Yuan Y Lu <yuan.y.lu@intel.com>
Tested-by: Yuan Y Lu <yuan.y.lu@intel.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>