mmc: block: remove unused struct mmc_card *card

It was never used and introduced a long standing compile
warning:

drivers/mmc/core/block.c: In function 'power_ro_lock_store':
drivers/mmc/core/block.c:191:19: warning: variable 'card' set but not
used [-Wunused-but-set-variable]

Remove it to fix the warning.

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