docs: networking: Update codeaurora references for rmnet

source.codeaurora.org is no longer accessible and so the reference link
in the documentation is not useful. Use iproute2 instead as it has a
rmnet module for configuration.

Fixes: ceed73a2 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Sean Tranchetti <quic_stranche@quicinc.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <quic_subashab@quicinc.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>