clk: mmp: fix wrong mask when calculate denominator

The code has typo when calculate denominator. It should use
den_mask instead of num_mask.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

diff --cc drivers/clk/mmp/clk-frac.c
Simple merge