When user attempted to perform a raw write using DFU (vide
dfu_fill_entity_mmc) with MMC interface not initialized before,
get_mmc_blk_size() reported invalid (zero) block size - it wasn't
possible to write ie. a new u-boot image.

This commit fixes that by initializing MMC device before use in
dfu_fill_entity_mmc().

While fixing initialization sequence, I had to change about half of
dfu_fill_entity_mmc's body, so I refactored it on the way to make it,
IMHO, considerably more comprehensible.

Being left as dead code, get_mmc_blk_size() was removed.

Tested on Samsung Goni.

Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Tom Rini <trini@ti.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>