ALSA: hda: Avoid NULL pointer dereference at snd_hdac_stream_start()

For ca0132 codec, azx_dev->stream is NULL during firmware loading.
Calling snd_hdac_get_stream_stripe_ctl unconditionally causes NULL
pointer dereference in that function.

Fixes: 9b6f7e7a

 ("ALSA: hda: program stripe bits for controller")
Signed-off-by: Mariusz Ceier <mceier+kernel@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>