mtd: docg4: ecc.read_page() returns 0 on uncorrectable errors

Currently the docg4's ecc.read_page() method returns -EBADMSG when
uncorrectable bitflips occur.  This is wrong; 0 should be returned in
this case.  An error code should only be returned by this method in the
case of a hardware error (probably -EIO).

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>