mtd: nand: brcmnand: Zero bitflip is not an error

A negative return value of brcmstb_nand_verify_erased_page() indicates a
real bitflip error of an erased page, and other return values (>= 0) show
the corrected bitflip number. Zero return value means no bitflip, but the
current driver code treats it as an error, and eventually leads to
falsely reported ECC error.

Fixes: 02b88eea ("mtd: brcmnand: Add check for erased page bitflip")
Signed-off-by: Albert Hsieh <wen.hsieh@broadcom.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>