mtd: nand: tango: Update ecc_stats.corrected

According to Boris, some user-space tools expect MTD drivers to
update ecc_stats.corrected, and it's better to provide a lower
bound than to provide no information at all.

Fixes: 6956e238

 ("mtd: nand: add tango NAND flash controller support")
Cc: stable@vger.kernel.org
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>