mmc:fix Call mmc_init() when executing mmc_get_dev()

mmc:fix Call mmc_init() when executing mmc_get_dev()

This code adds call to mmc_init(), for partition related commands (e.g.
fatls, fatinfo etc.).

It is safe to call mmc_init() multiple times since mmc->has_init flag
prevents from multiple initialization.

The FAT related code calls get_dev high level method and then uses
elements from mmc->block_dev, which is uninitialized until the mmc_init
(and thereof mmc_startup) is called.

This problem appears on boards, which don't use mmc as the default
place for envs

Signed-off-by: Lukasz Majewski <[email protected]>
Signed-off-by: Kyungmin Park <[email protected]>
Cc: Andy Fleming <[email protected]>
Signed-off-by: Andy Fleming <[email protected]>