RAS/CEC: Correct ce_add_elem()'s returned values

ce_add_elem() uses different return values to signal a result from
adding an element to the collector. Commit in Fixes: broke the case
where the element being added is not found in the array. Correct that.

 [ bp: Rewrite commit message, add kernel-doc comments. ]

Fixes: de0e0624

 ("RAS/CEC: Check count_threshold unconditionally")
Signed-off-by: William Roche <william.roche@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/1617722939-29670-1-git-send-email-william.roche@oracle.com