ARM: SAMSUNG: using vsnprintf instead of vsprintf for the limit buffer length 256

the buff is 256 limited, so need use vsnprintf instead of vsprintf

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: Ben Dooks <ben@fluff.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>