ALSA: line6: Abort if inconsistent usbdev is found at disconnect

It's utterly unsafe to proceed further the disconnect procedure if the
assigned usbdev is inconsistent with the expected object.  Better to
put a WARN_ON() for more cautions and abort immediately.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>