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.

Link: https://lore.kernel.org/all/2024100904-CVE-2024-47663-9bdc@gregkh/
Fixes: ea70758 ("Staging: IIO: DDS: AD9832 / AD9835 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/20241022134354.574614-1-quzicheng@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Zicheng Qu

drivers/staging/iio/frequency/ad9832.c

@@ -129,12 +129,15 @@ static unsigned long ad9832_calc_freqreg(unsigned long mclk, unsigned long fout)
 static int ad9832_write_frequency(struct ad9832_state *st,
 				  unsigned int addr, unsigned long fout)
 {
+	unsigned long clk_freq;
 	unsigned long regval;
 
-	if (fout > (clk_get_rate(st->mclk) / 2))
+	clk_freq = clk_get_rate(st->mclk);
+
+	if (!clk_freq || fout > (clk_freq / 2))
 		return -EINVAL;
 
-	regval = ad9832_calc_freqreg(clk_get_rate(st->mclk), fout);
+	regval = ad9832_calc_freqreg(clk_freq, fout);
 
 	st->freq_data[0] = cpu_to_be16((AD9832_CMD_FRE8BITSW << CMD_SHIFT) |
 					(addr << ADD_SHIFT) |