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>