media: siano: remove redundant null pointer checks in cec_devnode_init()

Since the debugfs_create_dir() never returns a null pointer, checking
the return value for a null pointer is redundant, Remove this check
since debugfs_create_file can handle IS_ERR pointers.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>