media: mc-device.c: fix memleak in media_device_register_entity

In media_device_register_entity, if media_graph_walk_init fails,
need to free the previously memory.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>