ALSA: asihpi - Fix uninitialized variable

Initialize prev_ctl properly before reference:
  sound/pci/asihpi/asihpi.c: In function ‘snd_card_asihpi_mixer_new’:
  sound/pci/asihpi/asihpi.c:2568:30: warning: ‘prev_ctl.dst_node_index’ may be used uninitialized in this function

Signed-off-by: Takashi Iwai <tiwai@suse.de>