mmc: block: move packed command struct init

By moving the mmc_packed_init() and mmc_packed_clean() into the
only file in the kernel where they are used, we save two exported
functions and can staticize those to the block.c file.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>