hww: fix digitalbitbox(1) support

regression from cea4238b81

```
975.04 | E | plugin.DeviceMgr | failed to create client for digitalbitbox at <REDACTED>: AttributeError("'NoneType' object has no attribute 'get_passphrase'")
Traceback (most recent call last):
  File "...\electrum\electrum\plugin.py", line 620, in list_pairable_device_infos
    soft_device_id = client.get_soft_device_id()
  File "...\electrum\electrum\plugins\hw_wallet\plugin.py", line 251, in get_soft_device_id
    root_fp = self.request_root_fingerprint_from_device()
  File "...\electrum\electrum\plugin.py", line 362, in wrapper
    return run_in_hwd_thread(partial(func, *args, **kwargs))
  File "...\electrum\electrum\plugin.py", line 352, in run_in_hwd_thread
    return func()
  File "...\electrum\electrum\plugins\hw_wallet\plugin.py", line 264, in request_root_fingerprint_from_device
    child_of_root_xpub = self.get_xpub("m/0'", xtype='standard')
  File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 115, in get_xpub
    reply = self._get_xpub(bip32_path)
  File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 110, in _get_xpub
    if self.check_device_dialog():
  File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 197, in check_device_dialog
    if not self.password_dialog(msg):
  File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 159, in password_dialog
    password = self.handler.get_passphrase(msg, False)
AttributeError: 'NoneType' object has no attribute 'get_passphrase'
```

electrum/plugins/digitalbitbox/digitalbitbox.py

@@ -125,6 +125,9 @@ class DigitalBitbox_Client(HardwareClientBase):
         else:
             raise Exception('no reply')
 
+    def get_soft_device_id(self):
+        return None
+
     def dbb_has_password(self):
         reply = self.hid_send_plain(b'{"ping":""}')
         if 'ping' not in reply: