Cleanup wwan-mm and use libmm-glib

This is in preparation for landing a cleaned up version of !1334 which properly moves the handling into the MM specific bits.

We so far avoided a dependency on libmm-glib but with the CBS support we use more bits from it and hence can switch the existing code over too. While at that modernize the style a bit (the wwan-mm interface is one of the oldest ones in phosh).

I'd be happy about a review.