do_readback not defined
Running 0.28.3, installed via pip, python 3.7.9.  wreg always causes the error:
```
pyocd> wreg GPIOA.BRR 0xF
writing 0xf to 0x48000028:32 (BRR)
Error: name 'do_readback' is not defined
```

diff --git a/pyocd/commands/commands.py b/pyocd/commands/commands.py
index 5f468fc4..4d7ada2d 100755
--- a/pyocd/commands/commands.py
+++ b/pyocd/commands/commands.py
@@ -319,7 +319,7 @@ class WriteRegCommand(RegisterCommandBase):
                     else:
                         raise exceptions.CommandError("too many dots")
                     self.context.target.flush()
-                    if do_readback:
+                    if self.do_readback:
                         self._dump_peripheral_register(p, r, True)
                 else:
                     raise exceptions.CommandError("invalid register '%s' for %s" % (subargs[1], p.name))
['pyocd/commands/commands.py']