ALSA: pcm: Add debug-print helper function

Adds a function getting the stream-name as a string for
a specific stream.

Signed-off-by: Ola Lilja <ola.o.lilja@stericsson.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>