net: rmnet_usb: don't access unet after freeing

When mux is enabled, unet is accessed after freeing it in
rmnet_usb_disconnect(). Fix it by caching the variables that
are required to retrieve unet from unet_list array before
freeing the unet.

CRs-Fixed: 562671
Change-Id: Id21a99db6b3f5da05e89e17f04c01202ea996d06
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>