mtd: atmel_nand: Do not warn on bitflips

When using multi-bit ECC, it is normal for the NAND Flash driver to
correct bit errors during the life of the product. Those errors will
only be cleared once a threshold has been reached, and corrections can
occur regularly before this.

Use only dev_dbg and not dev_info to report the bitflips, to keep the
system log clean when everything works correctly.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>