net: rmnet_data: Clear VNDs upon physical device unregistration

Clear out VNDs which have their egress device pointing to an interface
which is trying to unregister from the network stack. Required to prevent
systems hangs on unexpected shutdown/reboot of the device.

CRs-Fixed: 638324
Change-Id: I406270fee9feb1f9673b3391ce51c11e8e6c9d81
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>