mtd: rawnand: brcmnand: fix status read of brcmnand_waitfunc

This change fixes an issue where an error return value may be mistakenly
used as NAND status.

Fixes: f504551b ("mtd: rawnand: Propagate error and simplify ternary operators for brcmstb_nand_wait_for_completion()")
Signed-off-by: david regan <dregan@broadcom.com>
Reviewed-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>