net: qualcomm: rmnet: Refactor the new rmnet dev creation

Data format can be directly set from rmnet_newlink() since the
rmnet real dev info is already available.

Since __rmnet_get_real_dev_info() is no longer used in rmnet_config.c
after removal of those functions, move content to
rmnet_get_real_dev_info().

__rmnet_set_endpoint_config() is collapsed into
rmnet_set_endpoint_config() since only mux_id was being set additionally
within it. Remove an unnecessary mux_id check.

Set the mux_id for the rmnet_dev within rmnet_vnd_newlink() itself.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Cc: Dan Williams <dcbw@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>