GraphicEQControl: only re-render the written slider on write completion

The partial-dict hardening in d632febf made set_value call
control.set_value() for every band unconditionally. A per-slider
write returns a single-key result dict, so completing one write
re-rendered all 10 sliders from setting._value. Any slider the user
had dragged but whose 0.5s debounce hadn't fired yet got reverted to
its old value — and the subsequent debounce then read that reverted
value and dropped the user's change.

Restore the original behavior: only set sliders present in the result
dict; for the rest, read stored.get() for the tooltip but leave the
widget alone. Keeps the KeyError-safe .get() from d632febf.

lib/solaar/ui/config_panel.py

@@ -611,9 +611,10 @@ class GraphicEQControl(MultipleControl):
         for vbox in self._items:
             item = vbox._setting_item
             v = value.get(int(item))
-            if v is None:
+            if v is not None:
+                vbox.control.set_value(v)
+            else:
                 v = stored.get(int(item), 0)
-            vbox.control.set_value(v)
             b += f"{str(item)}: ({str(v)}) "
         lbl_text = ngettext("%d value", "%d values", n) % n
         self._button.set_label(lbl_text)