tinyalsa: add pcm_get_subdevice()

The users of tinyalsa had no way of knowing on which subdevice a stream had
been created.  A new API, "unsigned int pcm_get_subdevice(struct *pcm)" returns
it.  This information is filled during the pcm_open()

Upstream:
https://github.com/tinyalsa/tinyalsa/pull/48/commits/4cddf19dfcf65e6fb1277a3230fd0fe509ec7e81



Change-Id: I5e9195b710792c4bdb060425477268def24f77ef
Signed-off-by: David Wagner <david.wagner@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>