mtd: nand: nandsim: fix spelling mistake: "weakpagess" -> "weakpages"

trivial fix to spelling mistake in NS_ERR error message

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>