ASoC: ssm2518: Fix trivial typo in checking tx_mask and rx_mask values

Otherwise, ssm2518_set_tdm_slot() always returns error if slots != 0.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>