Fix a let hunk from the blk_mq_alloc_disk conversion.

Fixes: 281ea6a5bfdc ("mmc: switch to blk_mq_alloc_disk")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210621080144.3655131-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>