hps_accel/stream.py: improve generated code
hps_accel/stream.py: improve generated code

Improve names of variables in generated code by correctly setting
src_loc_at.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground