ASoC: Tegra: Fix compile when debugfs not enabled

The prototype of the inline dummy version of tegra_i2s_debug_add
was not consistent with the real version.

Reported-by: Rhyland-Klein <rklein@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>