ixgbe: do not clear FCoE DDP error status for received ABTS

The ddp->err is initialized to be 1 to make sure outstanding DDP context is
guaranteed to be invalidated when HW is not auto-invalidating it. However,
in case of receiving ABTS response for a DDPed I/O, the ddp->err was cleared,
bypassing the invalidating of the DDP context from upper protocol stack when
ixgbe_fcoe_ddp_put() is called. This bug is fixed here by updating the error
only when FCP_RSP is received.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>