mmc: block: Remove use of mmc_blk_set_blksize

According to the specifications for SD and (e)MMC default
blocksize (named BLOCKLEN in Spec.) must always be 512
bytes. Since we hardcoded to always use 512 bytes, we do
not explicitly have to set it. Future improvements should
potentially make it possible to use a greater blocksize
than 512 bytes, but until then let's skip this.

Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Reviewed-by: Subhash Jadavani <subhashj@codeauora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>