ALSA: hdac: fix the spbmaxfifo API

spbmaxfifo API is actually a query function not a set function so
name it snd_hdac_ext_stream_get_spbmaxfifo()

Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>