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: 5a042273

 ("drm/panel: Add ilitek ili9341 panel driver")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://lore.kernel.org/r/20240425142706.2440113-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240425142706.2440113-4-andriy.shevchenko@linux.intel.com