alsa_sound: fix watchdog error issue

- Issue happens when pcm_open fails after routing is done.
- Currently HAL returns zero bytes when pcm_open fails,
  Return actual bytes so that audioflinger can drop the buffer.

Bug-id: 7253359
Change-Id: I5d989539b0f9252577dc81a3ba34d467758bf717
Signed-off-by: Iliyan Malchev <malchev@google.com>