drm/rockchip/dsi: fix handling mipi_dsi_pixel_format_to_bpp result

The function can return negative value so it should be assigned to signed
variable.

The problem has been detected using patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>

drivers/gpu/drm/rockchip/dw-mipi-dsi.c

@@ -461,10 +461,11 @@ static int dw_mipi_dsi_phy_init(struct dw_mipi_dsi *dsi)
 
 static int dw_mipi_dsi_get_lane_bps(struct dw_mipi_dsi *dsi)
 {
-	unsigned int bpp, i, pre;
+	unsigned int i, pre;
 	unsigned long mpclk, pllref, tmp;
 	unsigned int m = 1, n = 1, target_mbps = 1000;
 	unsigned int max_mbps = dptdin_map[ARRAY_SIZE(dptdin_map) - 1].max_mbps;
+	int bpp;
 
 	bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
 	if (bpp < 0) {