ASoC: sn95031: Fix the logic to find free channel

In the case of no free channel available,
current implementation returns 0 instead of negative errno.

This patch fixes the logic to return -EINVAL if no free channel available.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Vinod Koul <vinod.koul@linux.intel.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>