mtd: nand: pxa3xx: read_page() returns max_bitflips

As per the ecc.read_page() prototype, we must return the maximum number
of bitflips that were corrected on any one region covering an ecc step.

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