net: websocket: Fix poll handling for offloaded sockets

ZFD_IOCTL_POLL_OFFLOAD operation needed special handling, as it needed
to modify the fds table for the offloaded implementation, overwriting
websocket file descriptors with the underlying offloaded ones. This is
only needed for the offloaded sockets, as the native implmentation use
POLL_PREPARE/UPDATE operations instead.

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