ti: Use devtype=mmc instead of setenv devtype mmc

If devtype variable is setted via setenv, then the following devtype=X style is
ignored. Currently, many u-boot commands use devtype variable in the latter
manner:

    mmc_boot=if mmc dev ${devnum}; then devtype=mmc; run scan_dev_for_boot_part; fi

Use devtype=mmc instead of setenv devtype mmc to avoid bugs with booting from
another devtype.

Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>