rapidio: fix an API misues when rio_add_net() fails

rapidio: fix an API misues when rio_add_net() fails

rio_add_net() calls device_register() and fails when device_register()
fails.  Thus, put_device() should be used rather than kfree().  Add
"mport->net = NULL;" to avoid a use after free issue.

Link: https://lkml.kernel.org/r/20250227073409.3696854-1-haoxiang_li2024@163.com
Fixes: e8de370188d0 ("rapidio: add mport char device driver")
Signed-off-by: Haoxiang Li <haoxiang_li2024@163.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>