fbdev: mb862xxfb: Use int type to store negative error codes

Change the 'ret' variable in of_platform_mb862xx_probe() from unsigned long
to int, as it needs to store either negative error codes or zero.

Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but can be confusing. Additionally, assigning negative error
codes to unsigned type may trigger a GCC warning when the -Wsign-conversion
flag is enabled.

No effect on runtime.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Helge Deller <deller@gmx.de>