mmc: queue: amend buffer swap for non-blocking transfer

In case both 'req' and 'mq->mqrq_prev->req' are null, there is no request
to be processed. That means there is no need to switch buffer.
Switching buffer is required only after finishing 'issue_fn'.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Per Forlin <per.forlin@stericsson.com>
Tested-by: Johan Rudholm <johan.rudholm@stericsson.com>
Signed-off-by: Chris Ball <cjb@laptop.org>