snd-fireworks: prevent WARN_ON() when remove() is executed

If MIDI stream exists, AMDTP stream is still running when PCM stream is going to stop.
Then remove() generates WARN_ON() in cmp_connection_destroy() because
snd_efw_destroy_pcm_devices() forget to call cmp_connection_break().

This commit take snd_efw_destroy_pcm_devices() to break CMP connection.
My series of patch for alsa-devel does not include this.

[PATCH 0/8] [RFC] new driver for Echo Audio's Fireworks based devices
http://mailman.alsa-project.org/pipermail/alsa-devel/2013-June/062614.html