ASoC: fsl: fsl_spdif: signedness bug in fsl_spdif_startup()

We need "i" to be signed or it leads to a forever loop in the error
handling.

Fixes: fa3be920 ('ASoC: fsl: fsl_spdif: Check for clk_prepare_enable() error')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>