ALSA: line6: Drop interface argument from private_init and disconnect callbacks

The interface argument is used just for retrieving the assigned
device, which can be already found in line6->ifcdev.  Drop them from
the callbacks.  Also, pass the usb id to private_init so that the
driver can deal with it there.  This is a preliminary work for the
further cleanup to move the whole allocation into driver.c.

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