sheep/oc: use strtoull instead of strtoul

Although long long data type is the same as long data type in 64-bit
platform, it is better to use only one of them. Since we use strtoull
in other place of the module, we should not use strtoul.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>