pcm: Do prepare action in pcm_open()

This eliminates the need for prepare in transfer functions
and pcm_start().

Signed-off-by: Ricardo Biehl Pasquali <pasqualirb@gmail.com>
diff --git a/src/pcm.c b/src/pcm.c
index 96a5788..f854a94 100644
--- a/src/pcm.c
+++ b/src/pcm.c
@@ -1129,6 +1129,10 @@
     }
 #endif
 
+    /* prepare here so the user does not need to do this later */
+    if (pcm_prepare(pcm))
+        goto fail;
+
     pcm->underruns = 0;
     return pcm;