video: mxsfb: fix clk_get_by_name() return value check

If clk_get_by_name() returns 0 it means it executed successfully while now
we consider it as an error. So let's check if return value is negative to
be an error. Otherwise this prevents "axi" and "disp_axi" to be enabled.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
diff --git a/drivers/video/mxsfb.c b/drivers/video/mxsfb.c
index 98d2965..5f85c0c 100644
--- a/drivers/video/mxsfb.c
+++ b/drivers/video/mxsfb.c
@@ -89,7 +89,7 @@
 	}
 
 	ret = clk_get_by_name(dev, "axi", &clk);
-	if (!ret) {
+	if (ret < 0) {
 		debug("%s: Failed to get mxs axi clk: %d\n", __func__, ret);
 	} else {
 		ret = clk_enable(&clk);
@@ -100,7 +100,7 @@
 	}
 
 	ret = clk_get_by_name(dev, "disp_axi", &clk);
-	if (!ret) {
+	if (ret < 0) {
 		debug("%s: Failed to get mxs disp_axi clk: %d\n", __func__, ret);
 	} else {
 		ret = clk_enable(&clk);