net: rmnet_usb: Fix assignment order of instances

Commit f0b04dee "net: rmnet_usb: Support multiple instances" changed
the storage of the pre-allocated control device structures by adding
them to a list. The problem with that approach is that across
disconnect/re-probe, each instance may not be re-associated with the
interface as used previously. This coupling must be ensured to allow
users of the control device files to continue to access the same
interfaces across subsystem restarts.

Fix this by creating the instances in a dynamically-allocated array.
Since we assume probe order is in increasing interface number per
USB device, use this to assign the same instance when the
device/interface is re-probed.

CRs-fixed: 453167
Change-Id: Ide20abdd7acf0194dfb9bbdd9d039a8888b23d93
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>