ixgbe: Do not return 0 in ixgbe_fcoe_ddp() upon FCP_RSP in DDP completion

ixgbe: Do not return 0 in ixgbe_fcoe_ddp() upon FCP_RSP in DDP completion

We return the ddp->len in ixgbe_fcoe_ddp() to indicate the length of data that
have been DDPed. However, it is possible that the length is 0, e.g., for SCSI
READ, the FCP_RSP may come back w/ SCSI status 0x28 as Task Set Full with no FCP
data for DDP. In ixgbe_fcoe_ddp(), we return 0 to indicate not passing DDPed
packets to upper layer. Therefore in the case of ddp->len being 0 upon FCP_RSP,
we do not want to return the 0 ddp->len as we want FCP_RSP to be always
delivered to the upper layer. This patch fixes this bug by setting rc only if
ddp->len is non-zero.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>