mmc: block: fixed NULL pointer dereference

We already check for ongoing async transfers when handling discard
requests, but not in mmc_blk_issue_flush().  This patch fixes that
omission.

Tested with an SDHCI controller and eMMC4.41.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Per Forlin <per.forlin@linaro.org>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>