net: websockets: Fix send with large payload

In case the payload and header size exceeded the network MTU size,
`websocket_send_msg()` would only send a part of the payload,
effectively leading to erronous results if called again to send the
rest. Fix the issue, by calling `sendmsg()` in a loop internally in case
it did not manage to send the entire websocket message in a single call.

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