ASoC: cs53l30: Fix a bug for TDM slot location validation

The maximum slot number of CS53L30 is 4 while it should support
the situation that's less than 4 channels based on the rx_mask.

So when the driver validates the last slot location, it should
check the last active slot instead of always the 4th one.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>