staging: line6: add bounds check in snd_toneport_source_put()

"source" comes from the user in snd_ctl_elem_write() so it needs to be
checked.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>