Merge #124

124: Fix UnboundLocalError in patch.py r=adamgreig a=jannic

Fixes this error message:
```
File "/home/jan/.local/lib/python3.7/site-packages/svdtools/patch.py", line 855, in derive_register
    description = rmod.get(description, None)
UnboundLocalError: local variable 'description' referenced before assignment
```

Co-authored-by: Jan Niehusmann <[email protected]>

Author: bors[bot]

svdtools/patch.py

@@ -852,7 +852,7 @@ def derive_register(self, rname, rmod):
         elif isinstance(rmod, dict):
             rderive = rmod["_from"]
             address_offset = rmod.get("addressOffset", None)
-            description = rmod.get(description, None)
+            description = rmod.get("description", None)
         else:
             raise SvdPatchError(f"derive: incorrect syntax for {rname}")
         rtag = parent.find(f"./register[name='{rname}']")