ASoC: stm32: sai: fix less than zero comparison on unsigned int

Currently, the comparison of div < 0 is always false because div is
an unsigned int. Fix this by making div an int.

Detected by CoverityScan, CID#1475309 ("Unsigned compared against 0")

Fixes: 8307b2af "(ASoC: stm32: sai: set sai as mclk clock provider")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/stm/stm32_sai_sub.c

@@ -336,8 +336,7 @@ static int stm32_sai_mclk_set_rate(struct clk_hw *hw, unsigned long rate,
 {
 	struct stm32_sai_mclk_data *mclk = to_mclk_data(hw);
 	struct stm32_sai_sub_data *sai = mclk->sai_data;
-	unsigned int div;
-	int ret;
+	int div, ret;
 
 	div = stm32_sai_get_clk_div(sai, parent_rate, rate);
 	if (div < 0)