brcm80211: fix possible memleak in brcmf_proto_msgbuf_attach

When brcmf_proto_msgbuf_attach fail and msgbuf->txflow_wq != NULL,
we should destroy the workqueue.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1595237765-66238-1-git-send-email-wangyufen@huawei.com