env_mmc: fix cannot save env issue

The env change its implementation after this log, while env mmc
didn't change it immediately, which cause issue. Follow to the
new style to fix it.

commit ea882baf
Author: Wolfgang Denk <wd@denx.de>
Date:   Sun Jun 20 23:33:59 2010 +0200

    New implementation for internal handling of environment variables.

Signed-off-by: Lei Wen <leiwen@marvell.com>