VIM3: mmc_get_env_dev correct non emmc boot sources

need return -1 if boot source is not EMMC or SD ( for example it will be
useful if we have multy env sources configuration and device was booted
from SPI flash and env need read from SPI not from mmc )

Signed-off-by: Artem Lapkin <art@khadas.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>