staging: line6: Call *_disconnect() via pointer

Which *_disconnect() to call on disconnect 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.

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>