SQL Server 2012 :: Call A Split Parameter Function

In t-sql 2012, the followinng sql works fine when I declare @reportID.

IF @reportID <> 0
BEGIN
SELECT 'Students report 1' AS selectRptName, 1 AS rptNumValue
UNION
SELECT 'Students report 2', 2
UNION

[code]...

However when I use the sql above in an ssrs 2012 report, the query does not work since the @reportID parameter can have 0, 1, or up to 200 values.Thus I am thinking of calling the following following function to split out the parameter values:

FUNCTION [dbo].[fn_splitString]
(
@listString VARCHAR(MAX)
)
RETURNS TABLE WITH SCHEMABINDING
AS
RETURN
(
SELECT SUBSTRING(l.listString, sn.Num + 1, CHARINDEX(',', l.listString, sn.Num + 1) - sn.Num - 1) _id
FROM (SELECT ',' + LTRIM(RTRIM(@listString)) + ',' AS listString) l
CROSS JOIN dbo.sequenceNumbers sn
WHERE sn.Num < LEN(l.listString)
AND SUBSTRING(l.listString, sn.Num, 1) = ','
)

GO

how to remove the @reportID <> 0 t-sql above and replace by calling the fn_splitString function?