evb_rk3399: revert CONFIG_SYS_MMC_ENV_DEV to 0

This was changed to 1 in commit 0717dde0, but a few months later,
commit 5f9411af swapped the order of eMMC and SD card by assigning
indexed aliases to `&sdhci` and `&sdmmc`.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
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Signed-off-by: Kever Yang <kever.yang@rock-chips.com>