mtd: nand: pxa3xx: re-read a page in raw mode on uncorrectable error

This only applies on BCH path.

When an empty page is read, it triggers an uncorrectable error. While
this is expected, the ECC engine might produce itself bitflips in the
read data under certain layouts. To overcome this situation, always
re-read the entire page in raw mode and check for the whole page to be
empty.

Also report the right number of bitflips if there are any.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>