env: mcc: fix compilation error with ENV_IS_EMBEDDED

When ENV_IS_EMBEDDED is enabled, ret is not defined but is used as a
return value in env_mmc_load().
This patch correct this issue and simplify the existing code, test only
one time #if defined(ENV_IS_EMBEDDED) and not in the function.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>