sysfs: disambiguate between "error code" and "failure" in comments

The sentence "Returns 0 on success or error" might be misinterpreted as
"the function will always returns 0", make it less ambiguous.

Also, use the word "failure" as the contrary of "success".

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>