clk: mmp: fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/clk/mmp/clk-frac.c:113:6: warning:
 symbol 'clk_factor_init' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Michael Turquette <mturquette@linaro.org>