ASoC: ssm2518: Use a signed return type for ssm2518_lookup_mcs()

The return type "unsigned int" was used by the ssm2518_lookup_mcs()
function even though it will eventually return a negative error code.
Improve this implementation detail by deletion of the type modifier then.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/ssm2518.c

@@ -309,7 +309,7 @@ static const struct snd_pcm_hw_constraint_list ssm2518_constraints_12288000 = {
 	.count = ARRAY_SIZE(ssm2518_rates_12288000),
 };
 
-static unsigned int ssm2518_lookup_mcs(struct ssm2518 *ssm2518,
+static int ssm2518_lookup_mcs(struct ssm2518 *ssm2518,
 	unsigned int rate)
 {
 	const unsigned int *sysclks = NULL;