RDMA/cxgb4: Ignore positive return values from cxgb4_*_send() functions

The cxgb4_*_send() functions return NET_XMIT_ values, which are
positive integers or negative errno values.  So don't treat positive
return values as an error.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>