staging: line6: eliminate useless index_out variable

Playback urbs use the index_out counter to decide which part of the
playback buffer to use. Since the urb already has a unique index in
range [0, LINE6_ISO_BUFFERS) there is no need to keep a separate
counter.

Use the urb index instead. This also eliminates the possibility of two
urbs using the same playback buffer space if they ever complete
out-of-order for some reason.

Signed-off-by: Stefan Hajnoczi <[e-mail]>
Signed-off-by: Markus Grabner <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/line6/pcm.h

···
 	unsigned char *buffer_in;
 
 	/**
-		 Temporary buffer index for playback.
-	*/
-	int index_out;
-
-	/**
 		 Previously captured frame (for software monitoring).
 	*/
 	unsigned char *prev_fbuf;

drivers/staging/line6/playback.c

···
 	urb_frames = urb_size / bytes_per_frame;
 	urb_out->transfer_buffer =
 	    line6pcm->buffer_out +
-	    LINE6_ISO_PACKETS * line6pcm->max_packet_size * line6pcm->index_out;
+	    index * LINE6_ISO_PACKETS * line6pcm->max_packet_size;
 	urb_out->transfer_buffer_length = urb_size;
 	urb_out->context = line6pcm;
-
-	if (++line6pcm->index_out == LINE6_ISO_BUFFERS)
-		line6pcm->index_out = 0;
 
 	if (test_bit(BIT_PCM_ALSA_PLAYBACK, &line6pcm->flags) &&
 	    !test_bit(BIT_PAUSE_PLAYBACK, &line6pcm->flags)) {