kernel: tick-sched: Fix compilation error on 32-bit

do_div() on 32-bit is changed into __div64_32(uint64_t *n, uint32_t
base), which requires its operand type to be specified.
Update the variable type to fix the compilation issue.

Change-Id: I82ad839c5a16096cfe000a65c9344b7d17214e29
Signed-off-by: Runmin Wang <runminw@codeaurora.org>