Fix handling change USB mode gcode command

Author: rechrtb

src/GCodes/GCodes2.cpp

@@ -3773,21 +3773,34 @@ bool GCodes::HandleMcode(GCodeBuffer& gb, const StringRef& reply) THROWS(GCodeEx
 					const size_t chan = gb.GetLimitedUIValue('P', NumSerialChannels);
 					bool hostMode = false;
 					bool seen = false;
-#if SUPPORT_USB_DRIVE
-					if (chan == 0 && gb.TryGetBValue('H', hostMode, seen)) // switch modes first
+
+					// Check if a USB mode change is requested through the 'H' parameter on channel 0.
+					// If not on channel 0, short-circuit evaluation means TryGetBValue is not performed
+					// and hostMode == false, result == GCodeResult:ok is unchanged.
+					if (chan == 0 && gb.TryGetBValue('H', hostMode, seen))
 					{
+#if SUPPORT_USB_DRIVE
 						if (!platform.SetUsbHostMode(hostMode, reply))
 						{
-							reply.printf("Unable to set to %s mode", hostMode ? "host" : "device");
+							reply.printf("Unable to set to USB %s mode", hostMode ? "host" : "device");
 							result = GCodeResult::error;
 						}
-					}
 #else
-					reply.printf("USB host mode unsupported");
-					result = GCodeResult::error;
+						// No support for changing to host mode; changing to device mode is ignored.
+						if (hostMode)
+						{
+							reply.printf("USB host mode unsupported");
+							result = GCodeResult::error;
+						}
 #endif
-					if (result == GCodeResult::ok && !hostMode) // switched to device mode with no error, handle other device mode args
+					}
+
+					// Handle the other parameters if specified along with the change to USB device mode,
+					// no USB mode change requested, or communication parameter configuration on other channels.
+					if (result == GCodeResult::ok && !hostMode)
 					{
+						seen = false;
+
 						GCodeBuffer * const gbp = (chan == 0) ? UsbGCode() : (chan == 1) ? AuxGCode() : Aux2GCode();
 						if (gb.Seen('B'))
 						{