env: mmc: Fix offset issue for env save

Fix the issue in commit 46c9016b (env: mcc: Drop unnecessary #ifdefs)
If CONFIG_SYS_REDUNDAND_ENVIRONMENT is not defined, the offset value
becomes undetermined, so write env to unexpected offset.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>