debugfs: update documented return values of debugfs helpers

Since commit ff9fb72b ("debugfs: return error values, not NULL")
these helper functions do not return NULL anymore (with the exception
of debugfs_create_u32_array()).

Fixes: ff9fb72b

 ("debugfs: return error values, not NULL")
Signed-off-by: Ronald Tschalär <ronald@innovation.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>