MLK-11911-1 mxc IPUv3: capture: Define div_ratio as type of int32_t

MLK-11911-1 mxc IPUv3: capture: Define div_ratio as type of int32_t

The local variable div_ratio could be less than zero, so let's define
it as type of int32_t instead of uint32_t.

This issue is reported by Coverity:
Unsigned compared against 0 (NO_EFFECT)
unsigned_compare: This less-than-zero comparison of an unsigned value
is never true. div_ratio < 0U.
	if (div_ratio > 0xFF || div_ratio < 0) {
		dev_dbg(ipu->dev, "value of pixel_clk extends normal range\n");
		return -EINVAL;
	}

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>