mtd: at91: atmel_nand: return bit flips for the PMECC read_page()

This patch fix pmecc's read_page() to return maximum number of bitflips, 0 if uncorrectable.

In the commit: 3f91e94f

 ("mtd: nand: read_page() returns max_bitflips ()"),
The ecc.read_page() is changed to return the maximum number of bitflips.
And when meet uncorrectable bitflips it needs to return 0.

See the comment in nand.h:
 * @read_page:	function to read a page according to the ECC generator
 *		requirements; returns maximum number of bitflips corrected in
 *		any single ECC step, 0 if bitflips uncorrectable, -EIO hw error

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>