drm/panel: ili9341: Use predefined error codes

In one case the -1 is returned which is quite confusing code for
the wrong device ID, in another the ret is returning instead of
plain 0 that also confusing as readed may ask the possible meaning
of positive codes, which are never the case there. Convert both
to use explicit predefined error codes to make it clear what's going
on there.

Fixes: 5a04227 ("drm/panel: Add ilitek ili9341 panel driver")
Signed-off-by: Andy Shevchenko <[email protected]>
Reviewed-by: Neil Armstrong <[email protected]>
Reviewed-by: Sui Jingfeng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Neil Armstrong <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

drivers/gpu/drm/panel/panel-ilitek-ili9341.c

@@ -422,7 +422,7 @@ static int ili9341_dpi_prepare(struct drm_panel *panel)
 
 	ili9341_dpi_init(ili);
 
-	return ret;
+	return 0;
 }
 
 static int ili9341_dpi_enable(struct drm_panel *panel)
@@ -726,7 +726,7 @@ static int ili9341_probe(struct spi_device *spi)
 	else if (!strcmp(id->name, "yx240qv29"))
 		return ili9341_dbi_probe(spi, dc, reset);
 
-	return -1;
+	return -ENODEV;
 }
 
 static void ili9341_remove(struct spi_device *spi)