mmc: simplify queue initialization

Merge three functions initializing the queue into a single one, and drop
an unused argument for it.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>