env_mmc.c: Allow environment to be used within SPL

Inside of SPL we only concern ourself with one MMC device, so instead of
being able to use CONFIG_SYS_MMC_ENV_DEV we need to use 0 in SPL.
Switch the code to use a 'dev' variable to facilitate this.

Signed-off-by: Tom Rini <trini@ti.com>