Changes

mgw: Split MDCX read-only validations into its own function

The SDP parsing is moved to a step beforehand, so that validation of
full message can be done in a subsequent step. That validation step is
moved into a function to give some air to handle_modify_con() and easily
spot the logic.

Change-Id: I469413dbe89cc4795deb50d76b434655cba9b776