mmc: fix null pointer use in mmc_blk_remove_req

A previous commit (fdfa20c1631210d0) reordered the shutdown sequence
in mmc_blk_remove_req. However, mmc_cleanup_queue is now called before
we get the card pointer, and mmc_cleanup_queue sets mq->card to NULL.

This patch moves the card pointer assignment before mmc_cleanup_queue.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>