net: rmnet_data: Enhancements to support SSR

Cleaned up refcount on get_dev_by_name.
Added new APIs to support cleanup of configuration and virtual devices.
Added explicit reference managment in association/un-association
and when setting/unsetting logical EP.

CRs-Fixed: 596227
Change-Id: Ic67bb649b0f0420d9a1e4bf5664ed63c0ff7d8bf
Signed-off-by: Harout Hadeshian <harouth@codeaurora.org>