staging: iio: frequency: ad9834: Validate frequency parameter value

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

staging: iio: frequency: ad9834: Validate frequency parameter value

In ad9834_write_frequency() clk_get_rate() can return 0. In such case

ad9834_calc_freqreg() call will lead to division by zero. Checking

'if (fout > (clk_freq / 2))' doesn't protect in case of 'fout' is 0.

ad9834_write_frequency() is called from ad9834_write(), where fout is

taken from text buffer, which can contain any value.

Modify parameters checking.

Found by Linux Verification Center (linuxtesting.org) with SVACE.