modem/{rmtfs,qrtr}: make modem work on downstream again, some upgrades

A while ago I wanted to test the modem on the bq-picmt (MSM8916) downstream port while trying to figure out why GPS was not working on mainline. Apparently all the modem stuff has been somewhat broken for quite some time now. This is an attempt to fix it properly. See commit log for details.

I have tested this on bq-picmt (downstream port of BQ Aquaris X5); the modem now works out of the box with oFono. Perhaps even Mobile Data could be working after https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1607. At the moment I'm unable to test it because oFono is not able to connect to the Internet on my BQ Aquaris X5 for some reason, while ModemManager works fine. ModemManager has no support for downstream.

Cc: @DolphinChips