md: fix the checking of wrong work queue

It should check md_rdev_misc_wq instead of md_misc_wq.

Fixes: cc1ffe61 ("md: add new workqueue for delete rdev")
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>