mtd: atmel_nand: return max bitflips in all sectors in pmecc_correction()

atmel_nand_pmecc_read_page() will return the total bitflips in this
page. This is incorrect.

As one nand page includes multiple ecc sectors, that will cause the
returned total bitflips exceed ecc capablity.

So this patch will make pmecc_correct() return the max bitflips of all
sectors in the page. That also makes atmel_nand_pmecc_read_page() return
the max bitflips.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>