IB/ocrdma_hw: remove unnecessary code in ocrdma_mbx_dealloc_lkey

Check on return value and goto label mbx_err are unnecessary.

Addresses-Coverity-ID: 1268780
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>