asoc: fix prepare error in pulseaudio loopback cases

In pulseaudio, prepare comes just after open for every
substream in loopback cases, but the 'audio_client'
is not allocated for the first substream in every pcm.
And the returned error causes the failure of pulseaudio
initialization for all loopback cases.

Change-Id: I7fb6c1f23b9cf2660e4dd519a731c7e13a809a4a
Signed-off-by: Guodong Hu <guodhu@codeaurora.org>