net: websocket: Make use of any leftover data after HTTP processing

In case HTTP client read out more data from a socket that it processed
it will indicate there is leftover data in the receive buffer available.
Make use of it at the websocket level, so that no data is lost. As we
reuse the same receive buffer in this case, it's only needed to update
the count variable to indicate how many bytes are available.

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