clk: fixed-factor: round_rate should use do_div

clk->rate = parent->rate / div * mult

The formula is OK. But it may overflow while we do operate with
unsigned long. So use do_div instead.

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: improved $SUBJECT]