net: qualcomm: rmnet: Allow to configure flags for existing devices

Add an option to configure the mux id, aggregation and commad feature
for existing rmnet devices. Implement the changelink netlink
operation for this.

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