kernel: sched.c: Fix datatype mismatch in comparision

All arguments comes from userspace has data type u32_t but
base.prio has data type of s8_t. Comparision between s8_t and u32_t
cannot be done. That's why typecast priority coming from userspace(prio)
to s8_t data type.

Signed-off-by: Punit Vara <punit.vara@intel.com>