mmc: add missing prototype for mmc_get_env_dev

This is a follow-up patch to e92029c0 and adds a prototype for
the weak mmc_get_env_dev function.

Cc: Tom Rini <trini@konsulko.com>
Cc: Stephen Warren <swarren@nvidia.com>

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>