MythMusic: fix playback of some radio stations

Make sure the maximum size of the QTCPSocket read buffer is set appropriately.

This seems to throttle the sending of readyRead() signals sent to us from
QTCPSocket better otherwise once we fill our buffer and don't read anything in the
readyRead() slot the QTCPSocket will eventually not send any more signals causing
us to run out of data.

Fixes #11872.