rtlwifi: Modify some USB de-initialize code.

Delete SET_USB_STOP macro and rtl_usb_deinit because
those are called twice in USB de-initialize routine.
Add some de-initialize workqueue function in USB disconnect routine.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>