mmc: queue: exclude asynchronous transfer for special request

Unlike normal r/w request, special requests(discard, flush)
is finished with a one-time issue_fn. Request change to
mqrq_prev makes unnecessary call.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Konstantin Dorfman <kdorfman@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>