We should call destroy_workqueue to destroy mlme_workqueue in error branch.

Fixes: ded845a7 ("ieee802154: Add CA8210 IEEE 802.15.4 device driver")
Signed-off-by: Liu Jian <liujian56@huawei.com>
Link: https://lore.kernel.org/r/20200720143315.40523-1-liujian56@huawei.comSigned-off-by: Stefan Schmidt <stefan@datenfreihafen.org>