Fix a typo in SYCL version of scan (AMReX-Codes#3757)

This is a small performance issue. With the typo, it always does a
memcpy from device to host to get the total sum even when it's told it's
not needed.

Src/Base/AMReX_Scan.H

@@ -400,7 +400,7 @@ T PrefixSum (N n, FIN && fin, FOUT && fout, TYPE type, RetSum a_ret_sum = retSum
 
 #ifndef AMREX_SYCL_NO_MULTIPASS_SCAN
     if (nblocks > 1) {
-        return PrefixSum_mp<T>(n, std::forward<FIN>(fin), std::forward<FOUT>(fout), type, retSum);
+        return PrefixSum_mp<T>(n, std::forward<FIN>(fin), std::forward<FOUT>(fout), type, a_ret_sum);
     }
 #endif