mwifiex: fix missing destroy_workqueue() on error in mwifiex_add_virtual_intf()

Add the missing destroy_workqueue() before return from
mwifiex_add_virtual_intf() in the error handling case.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>