drm/panel: novatek-nt35560: Fix invalid return value

Fix bug in nt35560_set_brightness() which causes the function to
erroneously report an error. mipi_dsi_dcs_write() returns either a
negative value when an error occurred or a positive number of bytes
written when no error occurred. The buggy code reports an error under
either condition.

Fixes: 8152c2bfd780 ("drm/panel: Add driver for Sony ACX424AKP panel")
Reviewed-by: Douglas Anderson <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Signed-off-by: Brigham Campbell <[e-mail]>
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/panel/panel-novatek-nt35560.c

···
 		par = 0x00;
 		ret = mipi_dsi_dcs_write(dsi, MIPI_DCS_WRITE_CONTROL_DISPLAY,
 					 &par, 1);
-		if (ret) {
+		if (ret < 0) {
 			dev_err(nt->dev, "failed to disable display backlight (%d)\n", ret);
 			return ret;
 		}