Revert "mdraid: call mdadm with "--wait-clean" instead of "-W""

This reverts commit bc4ce61efc.

Read wrong man page section. Got confused by
https://github.com/dracutdevs/dracut/issues/342

modules.d/90mdraid/mdraid-waitclean.sh

@@ -13,12 +13,12 @@ if getargbool 0 rd.md.waitclean; then
             continue
         fi
         info "Waiting for $md to become clean"
-        mdadm $_offroot --wait-clean "$md" >/dev/null 2>&1
+        mdadm $_offroot -W "$md" >/dev/null 2>&1
     done
 
     for md in $containers; do
         info "Waiting for $md to become clean"
-        mdadm $_offroot --wait-clean "$md" >/dev/null 2>&1
+        mdadm $_offroot -W "$md" >/dev/null 2>&1
     done
 
     unset containers udevinfo _offroot