sched/walt: use do_div instead of division operator

Use do_div() instead of "/" operator to fix undefined references to
"__aeabi_uldivmod" build error for ARCH=arm.

Also in TP_fast_assign(), along with do_div() usage,  replace "," with
";" which would have resulted in a syntax error (!), because
'#define TP_fast_assign(args...) args' would have stripped off the ","
and left white space between these two assignments after CPP phase.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[jstultz: Cherry-picked from common/android-3.18]
Signed-off-by: John Stultz <john.stultz@linaro.org>