broadband-modem-qmi: choose endpoint number based on data port

This is an extension of !853 (merged) that makes multiplexing/multiple bearers work on newer firmware versions with BAM-DMUX (e.g. MSM8909, see #610). Currently it works only with the following assumptions:

• Multiple network interfaces were created with DPM Open Port before starting ModemManager.
• The WDA data format is set correctly by default (to Raw-IP). WDA Set Data Format is currently still done only for the primary data port but not all the additional ones. (In practice this is not a problem, actually the MSM8909 firmware I have does not seem to support Ethernet mode anymore at all...)

The commit log includes some independent cleanup and fixes. In particular, I realized that multiplexing does not work at all for BAM-DMUX at the moment because the same WDS client is reused for all bearers. I'm not sure how it passed my testing on MSM8916 before, I guess I did not test properly after we decided to use a single QMI port instead of the separate ones. 😅

See commit messages for details. Note that this MR includes the commit from !853 (merged) to avoid conflicts. It is kind of independent though: In case there are any problems with this MR the other MR still makes sense without this one since it allows basic functionality at least. :) (The other MR was merged.)