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 <stefanha@gmail.com>
Signed-off-by: Markus Grabner <grabner@icg.tugraz.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>