net: websockets: Fix websocket_send_msg return code

When the websocket_prepare_and_send is called from websocket_send_msg,
the header length is subtracted to retrieve the transmitted payload length.
Make an exclutsion to prevent the return code of
websocket_prepare_and_send being modified in case of 0 or a negative
return code.
This avoid confusion with modificated error codes

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>