staging: line6: Call *_process_message() via pointer

staging: line6: Call *_process_message() via pointer

Which *_process_message() function (if any) to call when data is
received is known at initialization.  Add a function pointer to the
`usb_line6' struct and use to call into the appropriate logic instead
of evaluating the conditional logic for each message.

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>