staging: iio: frequency: ad9832: fix division by zero in ad9832_calc_freqreg()

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

staging: iio: frequency: ad9832: fix division by zero in ad9832_calc_freqreg()

In the ad9832_write_frequency() function, clk_get_rate() might return 0.

This can lead to a division by zero when calling ad9832_calc_freqreg().

The check if (fout > (clk_get_rate(st->mclk) / 2)) does not protect

against the case when fout is 0. The ad9832_write_frequency() function

is called from ad9832_write(), and fout is derived from a text buffer,

which can contain any value.