qt new_channel_dialog: fix regression: oepn chan with "max" amt

```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\channels_list.py", line 377, in new_channel_with_warning
	self.new_channel_dialog()
  File "...\electrum\electrum\gui\qt\channels_list.py", line 399, in new_channel_dialog
	return d.run()
  File "...\electrum\electrum\gui\qt\new_channel_dialog.py", line 133, in run
	if self.min_amount_sat and funding_sat < self.min_amount_sat:
TypeError: '<' not supported between instances of 'str' and 'int'
```

electrum/gui/qt/new_channel_dialog.py

@@ -130,9 +130,10 @@ class NewChannelDialog(WindowModalDialog):
             funding_sat = self.amount_e.get_amount()
         if not funding_sat:
             return
-        if self.min_amount_sat and funding_sat < self.min_amount_sat:
-            self.window.show_error(_('Amount too low'))
-            return
+        if funding_sat != '!':
+            if self.min_amount_sat and funding_sat < self.min_amount_sat:
+                self.window.show_error(_('Amount too low'))
+                return
         if self.network.channel_db:
             connect_str = str(self.remote_nodeid.text()).strip()
         else: