staging: r8188eu: usb_vendor_req_mutex is not used

usb_vendor_req_mutex in struct dvobj_priv is not used.
Remove it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220206175925.461426-2-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>