Guest writes to a readonly register trigger the assert in
intel_hda_reg_write().  Add a check and just ignore them.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1628433Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181123063957.9515-1-kraxel@redhat.com