gs_usb, interface: support this interface on command-line with e.g. can.logger by treating channel like index when all other arguments are missing

Author: BenGardiner

can/interfaces/gs_usb.py

@@ -38,6 +38,9 @@ def __init__(
                 "index and bus/address cannot be used simultaneously"
             )
 
+        if index is None and address is None and bus is None:
+            index = channel
+
         self.index = None
         if index is not None:
             devs = GsUsb.scan()

doc/interfaces/gs_usb.rst

@@ -8,13 +8,14 @@ and candleLight USB CAN interfaces.
 
 Install: ``pip install "python-can[gs_usb]"``
 
-Usage: pass device ``index`` (starting from 0) if using automatic device detection:
+Usage: pass device ``index`` or ``channel`` (starting from 0) if using automatic device detection:
 
 ::
 
     import can
 
     bus = can.Bus(interface="gs_usb", channel=dev.product, index=0, bitrate=250000)
+    bus = can.Bus(interface="gs_usb", channel=0, bitrate=250000)  # same
 
 Alternatively, pass ``bus`` and ``address`` to open a specific device. The parameters can be got by ``pyusb`` as shown below: