Check on return values and goto label mbx_err are unnecessary.

Addresses-Coverity-ID: 1271151 ("Identical code for different branches")
Addresses-Coverity-ID: 1268788 ("Identical code for different branches")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>