mmc: fix env in mmc with redundant compile error

mmc: fix env in mmc with redundant compile error

The commit d196bd8 (env_mmc: add support for redundant environment)
introduce the following compile error when enable redundant
environment support with MMC
---8<---
env_mmc.c:149: error: 'env_t' has no member named 'flags'
env_mmc.c:248: error: 'env_t' has no member named 'flags'
env_mmc.c:248: error: 'env_t' has no member named 'flags'
env_mmc.c:250: error: 'env_t' has no member named 'flags'
env_mmc.c:250: error: 'env_t' has no member named 'flags'
env_mmc.c:252: error: 'env_t' has no member named 'flags'
env_mmc.c:252: error: 'env_t' has no member named 'flags'
env_mmc.c:254: error: 'env_t' has no member named 'flags'
env_mmc.c:254: error: 'env_t' has no member named 'flags'
env_mmc.c:267: error: 'env_t' has no member named 'flags'
make[1]: *** [env_mmc.o] Error 1
--->8---

Add this patch to fix it

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Michael Heimpold <mhei@heimpold.de>
Signed-off-by: Andy Fleming <afleming@freescale.com>