Fix gs_usb shutdown to always call parent BusABC.shutdown()

BusABC has a class-level _is_shutdown = True attribute. When __init__
was not called (as in test mocks), GsUsbBus.shutdown() resolved this
class attribute and returned early, never calling super().shutdown().

Restructure shutdown() to always call super().shutdown(), using the
pre-call _is_shutdown state only to guard interface-specific cleanup.

Co-authored-by: BenGardiner <243321+BenGardiner@users.noreply.github.com>

Author: Copilot

can/interfaces/gs_usb.py

@@ -68,7 +68,6 @@ def __init__(
         :param can_filters: not supported
         :param bitrate: CAN network bandwidth (bits/s)
         """
-        self._is_shutdown = False
         if (index is not None) and ((bus or address) is not None):
             raise CanInitializationError(
                 "index and bus/address cannot be used simultaneously"
@@ -194,10 +193,11 @@ def _recv_internal(self, timeout: float | None) -> tuple[can.Message | None, boo
         return msg, False
 
     def shutdown(self):
-        if self._is_shutdown:
+        already_shutdown = self._is_shutdown
+        super().shutdown()
+        if already_shutdown:
             return
 
-        super().shutdown()
         self.gs_usb.stop()
         if self._index is not None:
             # Avoid errors on subsequent __init() by repeating the .scan() and .start() that would otherwise fail
@@ -211,4 +211,3 @@ def shutdown(self):
                     gs_usb.stop()
                 except usb.core.USBError:
                     pass
-        self._is_shutdown = True