RDMA/nes: Check for zero STag

STags are generated randomly but the driver does not correctly prevent
a zero STag.  Using STag zero is privileged and causes a user space
application to fail.  This change prevents the driver from trying to
allocate a zero STag.

Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>