ALSA: line6: Clear prev_fbuf and prev_fsize properly

Clearing prev_fsize in line6_pcm_acquire() is pretty racy.
This can be called at any time while the stream is being played.
Rather better to clear prev_fbuf and prev_fsize at the proper place
like the stream stop for capture, and just after copying the monitor /
impulse data inside the spinlock.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/line6/playback.c b/sound/usb/line6/playback.c
index b4a26d0..2422659 100644
--- a/sound/usb/line6/playback.c
+++ b/sound/usb/line6/playback.c
@@ -166,9 +166,7 @@
 		struct usb_iso_packet_descriptor *fout =
 		    &urb_out->iso_frame_desc[i];
 
-		if (line6pcm->flags & LINE6_BITS_CAPTURE_STREAM)
-			fsize = line6pcm->prev_fsize;
-
+		fsize = line6pcm->prev_fsize;
 		if (fsize == 0) {
 			int n;
 
@@ -263,6 +261,8 @@
 						   line6pcm->volume_monitor,
 						   bytes_per_frame);
 		}
+		line6pcm->prev_fbuf = NULL;
+		line6pcm->prev_fsize = 0;
 	}
 	spin_unlock(&line6pcm->in.lock);