ext/alsa/gstalsamixer.c: Use snd_mixer_selem_set_{playback|capture}_volume_all()...

ext/alsa/gstalsamixer.c: Use snd_mixer_selem_set_{playback|capture}_volume_all() if the volume is the same for all ch...

Original commit message from CVS:
Patch by: Bastien Nocera <hadess at hadess net>
* ext/alsa/gstalsamixer.c: (gst_alsa_mixer_get_volume),
(check_if_volumes_are_the_same), (gst_alsa_mixer_set_volume):
Use snd_mixer_selem_set_{playback|capture}_volume_all() if
the volume is the same for all channels. This works around
some problem in alsa that leaves us with inconsistent state
for some reason (#486840).