alsabat: add default device name for playback and capture

alsabat: add default device name for playback and capture

Add default name for the playback and capture devices, in case
they were not set by user through '-D', '-P' or '-C' options.
Previously, if no device be specified, the alsabat will start
a playback thread and a capture thread, and then exit the
threads with error log.
If only one of playback and capture is specified, the alsabat
will work on single line mode as before, where only one thread
(playback or capture) will be started.
The patch was tested on Ubuntu and Chrome OS.

Signed-off-by: Lu, Han <han.lu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>