env: mmc: print MMC device being read

env: mmc: print MMC device being read

This prints the MMC device being read similar to how we print the MMC
device we write to when e.g. calling saveenv.

One of the side effects is that the boot log now shows from which MMC
device the env was loaded:

Loading Environment from MMC... Reading from MMC(1)... OK

This is useful to identify which MMC device the environment was loaded
from for boards where there are more than one (e.g. eMMC and SD card)
without adding some debug messages manually.

Sadly, there's no way to know which of the default or redundant
environment is being read from env_mmc_load before env_import_redund is
called so it is printing a bit later (and possibly after error/warning
messages).

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>