cm-t54: fix eMMC boot mode check

Boot from eMMC boot partition corresponds to BOOT_DEVICE_MMC2
omap_bootmode, while BOOT_DEVICE_MMC2_2 corresponds to the user
data partition boot.

Fix mmc_get_env_part() boot mode check to use a correct value.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>