ALSA: hda - Fix a sparse warning about snd_ctl_elem_iface_t

The knew->iface field is in snd_ctl_elem_iface_t, which is with
__bitwise, hence it can't be converted implicitly from integer.
Give an explicit cast for the invalid type.

Spotted by sparse:
  sound/pci/hda/hda_codec.c:3280:25: warning: restricted snd_ctl_elem_iface_t degrades to integer

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