[ARM] S3C24XX: Fix sparse warnings in pwm.c

Fix the following sparse warnings in arch/arm/plat-s3c24xx/pwm.c:

warning: symbol 's3c_device_timer' was not declared. Should it be static?
warning: symbol 'pwm_calc_tin' was not declared. Should it be static?

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