ALSA: line6: Sync PCM stop at disconnect

Call line6_pcm_disconnect() at disconnect to make sure that all URBs
are cleared.  Also reduce the superfluous snd_pcm_stop() calls from
the function (and remove the unused function) since the streams are
guaranteed to be stopped at this point via snd_card_disconnect().

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>