fix define name to match mxm2 API

cmr=v1.7.4:reviewer=ompi-rm1.7

This commit was SVN r29311.

oshmem/mca/atomic/mxm/atomic_mxm_fadd.c

@@ -113,7 +113,7 @@ int mca_atomic_mxm_fadd(void *target,
         sreq.opcode = MXM_REQ_OP_ATOMIC_ADD;
 #else
         sreq.flags = 0;
-        sreq.opcode = MXM_REQ_OP_ATOMIC_ADD_SYNC;
+        sreq.opcode = MXM_REQ_OP_ATOMIC_FADD;
 #endif
     } else {
         sreq.base.data.buffer.ptr = prev;