mmc: block: rename data to blkdata

The struct mmc_blk_request contains an opaque void *data that
is actually only used to store a pointer to a per-request
struct mmc_blk_data. This is confusing, so rename the member
to blkdata and forward-declare the block.c local struct.

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