staging: rtl8188eu: Remove continual_urb_error from struct dvobj_priv

rtw_inc_and_chk_continual_urb_error() was only user of continual_urb_error,
which has been removed.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>