coresight: Fix loss of connection info when a module is unloaded

child_fwnode should be a read only property based on the DT or ACPI. If
it's cleared on the parent device when a child is unloaded, then when
the child is loaded again the connection won't be remade.

child_dev should be cleared instead which signifies that the connection
should be remade when the child_fwnode registers a new coresight_device.

Similarly the reference count shouldn't be decremented as long as the
parent device exists. The correct place to drop the reference is in
coresight_release_platform_data() which is already done.

Reproducible on Juno with the following steps:

  # load all coresight modules.
  $ cd /sys/bus/coresight/devices/
  $ echo 1 > tmc_etr0/enable_sink
  $ echo 1 > etm0/enable_source
  # Works fine ^

  $ echo 0 > etm0/enable_source
  $ rmmod coresight-funnel
  $ modprobe coresight-funnel
  $ echo 1 > etm0/enable_source
  -bash: echo: write error: Invalid argument

Fixes: 37ea1ffd ("coresight: Use fwnode handle instead of device names")
Fixes: 2af89eba ("coresight: Clear the connection field properly")
Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230425143542.2305069-2-james.clark@arm.com

drivers/hwtracing/coresight/coresight-core.c

@@ -1419,13 +1419,8 @@ static int coresight_remove_match(struct device *dev, void *data)
 		if (csdev->dev.fwnode == conn->child_fwnode) {
 			iterator->orphan = true;
 			coresight_remove_links(iterator, conn);
-			/*
-			 * Drop the reference to the handle for the remote
-			 * device acquired in parsing the connections from
-			 * platform data.
-			 */
-			fwnode_handle_put(conn->child_fwnode);
-			conn->child_fwnode = NULL;
+
+			conn->child_dev = NULL;
 			/* No need to continue */
 			break;
 		}