staging: line6: fix memory leak in .hw_params()

The .hw_params() pcm callback can be invoked multiple times in a row.
Ensure that the URB data buffer is only allocated once.

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>