dm: remove unnecessary assignment statement in alloc_dev()

[ Upstream commit 99f4f5bc ]

Fixes: 74fe6ba9 ("dm: convert to blk_alloc_disk/blk_cleanup_disk")
Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>