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>