net: websocket: Fix ioctl implementation for websocket

The websocket implementation of ioctl wrongly passed websocket context
to the undrelying `ioctl` implementation instead of the context of the
underlying socket.

Additionally, currentl implementation used the vtable of the native
socket implementation unconditionally, making it unusable with an
offloaded underlying socket.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>