ASoC: samsung-i2s: Check secondary DAI exists before referencing

In a couple of places the driver is missing a check to ensure there is a
secondary DAI before it de-references the pointer to it, causing a null
pointer de-reference. This patch adds a check to avoid this.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org