staging: line6: fix playback urb transfer buffer calculation

The playback urb transfer buffer calculation does not factor in
LINE6_ISO_PACKETS.  Buffer memory is organized like this in the driver:

  Buffer 0                 Buffer 1                 ...
  [Packet 0, Packet 1, ...][Packet 0, Packet 1, ...][Packet 0, ...]

However, we're lucky that LINE6_ISO_PACKETS is currently defined as 1 so
this patch does not change any behavior.  It's still worth including
this fix in case the LINE6_ISO_PACKETS value is changed in the future.

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>