staging: line6: Pass *_process_message() `usb_line6' pointers

Casting the `struct usb_line6' pointer at the call point makes the code
difficult to read.  This is substantially cleaned up by moving the cast
into the callees.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>