Do not stop boot as soon as an ECC error is detected. Only stop boot for
uncorrectable ECC errors.

This fixes boards no longer booting after some time because a NAND Flash bit has
flipped.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>