mtd: rawnand: pl353: Use int type to store negative error codes

mtd: rawnand: pl353: Use int type to store negative error codes

Change the 'ret' variable from u32 to int in pl35x_nand_probe() to store
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: Miquel Raynal <miquel.raynal@bootlin.com>