ALSA: hda/tegra - Fix build error and warning

I seem to have failed to run the build test properly...
   sound/pci/hda/hda_tegra.c: In function 'hda_tegra_dev_free':
   sound/pci/hda/hda_tegra.c:310:20: error: 'bus' undeclared (first use in this function)
         snd_hdac_bus_exit(bus);

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>