net: qualcomm: rmnet: Export mux_id and flags to netlink

Define new netlink attributes for rmnet mux_id and flags. These
flags / mux_id were earlier using vlan flags / id respectively.
The flag bits are also moved to uapi and are renamed with
prefix RMNET_FLAG_*.

Also add the rmnet policy to handle the new netlink attributes.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>