[dvsim] Correct bug in regression creation in dvsim's Modes.py

The create_regressions function is a static method, so there's no
'self' available. I think this should have been called on the
regression object.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/util/dvsim/Modes.py b/util/dvsim/Modes.py
index 3a757c9..e553aca 100644
--- a/util/dvsim/Modes.py
+++ b/util/dvsim/Modes.py
@@ -509,7 +509,7 @@
                 # which case, skip
                 if run_mode_obj.name in sim_cfg.en_run_modes:
                     continue
-                self.run_opts.extend(run_mode_obj.run_opts)
+                regression_obj.run_opts.extend(run_mode_obj.run_opts)
 
             # Unpack tests
             if regression_obj.tests == []: