ALSA: hda: assign polling_mode after azx_bus_init

We will move the polling_mode flag from struct azx to struct hdac_bus,
and the flag should be assigned after bus init.

Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>