mmc: block: Fix an invalid use of req->cmd_flag in DISCARD operation

When handling a DISCARD operation, the MMC request data structure may be
freed in memory. Therefore, it can't be used to retrieve the cmd_flags
for checking if MMC_REQ_NOREINSERT_MASK is set:
(!(mq->mqrq_cur->req->cmd_flags & MMC_REQ_NOREINSERT_MASK)))).

To prevent the issue we should use the local variable of cmd_flags.

Change-Id: Idef53d5bd66fa6f1faaf79644c8efb5177c75e89
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>