Use snprintf to replace sprintf.

Coverity log:
"
Unbounded source buffer (STRING_SIZE)
string_size: Passing string init_val of unknown size to sprintf.
"

Reported-by: Coverity
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>