mtd: nand: pxa3xx: Clean pxa_ecc_init() error handling

Let's make pxa_ecc_init() return a negative errno on error or zero
if succesful, which is standard kernel practice. Also, report the
selected ECC strength and step size, which is important information.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>