mailbox: mailbox-test: free channels on probe error

mailbox: mailbox-test: free channels on probe error

On probe error, free the previously obtained channels. This not only
prevents a leak, but also UAF scenarios because the client structure
will be removed nonetheless because it was allocated with devm.

Link: https://sashiko.dev/#/patchset/20260327151217.5327-2-wsa%2Brenesas%40sang-engineering.com
Fixes: 8ea4484d0c2b ("mailbox: Add generic mechanism for testing Mailbox Controllers")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>