mtd: nand-bb: use list_for_each_entry_safe

We need list_for_each_entry_safe in dev_remove_bb_dev since the
list entries are removed during iteration over the list.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: Alexander Aring <alex.aring@gmail.com>
mtd: nand-mxs: Return bitflips

The read_page hook shall return the number of bitflips. Implement
that.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>