dfu: mmc: raw data write fix

dfu: mmc: raw data write fix

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 <[email protected]>
Signed-off-by: Kyungmin Park <[email protected]>
Acked-by: Lukasz Majewski <[email protected]>
Acked-by: Tom Rini <[email protected]>
Cc: Minkyu Kang <[email protected]>