drivers: net: rmnet: Free port after netdev unreg

Previously rmnet port would be freed before netdevice unregister
queue. This would cause an issue if any dependent drivers relied on
rmnet_port.

This change switches the order and unregisters in context instead
of a unregister queue to ensure rmnet_port is freed after all netdev_cb
notifications.

Change-Id: Id8fdae83bddc5432501ee53142ece2695e80ab64
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>