env_nand: Use nand_read_skip_bad instead of nand_read

The nand-read function returns an error code if correctable errors have occurred.
This is not desirable, since the errors have been corrected!

This patch switches to the nand_read_skip_bad function which does not
return an error code if the errors are correctable.

Signed-off-by: Steve Sakoman <steve.sakoman@linaro.org>
Acked-by: Scott Wood <scottwood@freescale.com>