net: qualcomm: rmnet: Allow configuration updates to existing devices

This allows the changelink operation to succeed if the mux_id was
specified as an argument. Note that the mux_id must match the
existing mux_id of the rmnet device or should be an unused mux_id.

Fixes: 1dc49e9d

 ("net: rmnet: do not allow to change mux id if mux id is duplicated")
Reported-and-tested-by: Alex Elder <elder@linaro.org>
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>