mmc: core: remove redundant memset of mmc_decode_cid

When initializing sd or sdio card, we get struct mmc_card
from mmc_alloc_card which allocates it by kzalloc. So we don't
need another memset while decoding cid.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>