pwm: mediatek: Prevent divide-by-zero in pwm_mediatek_config()

In the Linux kernel, the following vulnerability has been resolved:

pwm: mediatek: Prevent divide-by-zero in pwm_mediatek_config()

With CONFIG_COMPILE_TEST && !CONFIG_HAVE_CLK, pwm_mediatek_config() has a

divide-by-zero in the following line:

do_div(resolution, clk_get_rate(pc->clk_pwms[pwm->hwpwm]));

due to the fact that the !CONFIG_HAVE_CLK version of clk_get_rate()

returns zero.

This is presumably just a theoretical problem: COMPILE_TEST overrides

the dependency on RALINK which would select COMMON_CLK. Regardless it's

a good idea to check for the error explicitly to avoid divide-by-zero.

Fixes the following warning:

drivers/pwm/pwm-mediatek.o: warning: objtool: .text: unexpected end of section

[ukleinek: s/CONFIG_CLK/CONFIG_HAVE_CLK/]