mtd: spi-nor: cadence-quadspi: fix spelling mistake: "Couldnt't" -> "Couldn't"

There is a spelling mistake in a dev_error message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>