net: rmnet_data: Handle device unregister notifications

Gracefully handle device unregister notifications. Cleans
up any logical endpoints configured on a physical devices
and then unassociates it. Required to prevent crash if
references are held too long.

CRs-Fixed: 596227
Change-Id: I02d08e07e74510b7a8dffbefa99e651e0100db23
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>