[ARM] S3C: Fix ADC driver sparse warning

The symbol 's3c_adc_try' in arch/arm/plat-s3c24xx/adc.c
does not need to be exported and thus should be static.

This fixes the following sparse warning:

adc.c:103:6: warning: symbol 's3c_adc_try' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>