[ARM] 3744/1: MMC: mmcqd gets stuck when block queue is plugged

Patch from Juha [\18êölä

When the block queue is plugged, mq->req must be set to NULL.
Otherwise mq->req might be left non-NULL, even though mmcqd is
not processing a request, thus preventing the MMC queue thread from
being woken up when new requests do arrive.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>