broadband-modem-mbim: skip switch() when selecting MBIM services
Having a switch() for the MBIM services when processing indications forces us to update it on every new MBIM service added to libmbim, because we build with -Wswitch-enum by default.
This warning type is extremely useful, and we should not stop using it, so let's simplify a bit the indication handling code and skip using a switch().
There are right now only 4 different service indications expected, so it shouldn't be a big deal.