The functions l1if_open_pdch and l1if_close_pdch have a misleading
naming since what they actually do is opening and closing the TRX since
they return and accept a context (obj) that is valid for a whole TRX.
This also explains why the other functions accept a timeslot as
parameter in addition to the context. Let's rename those functions so
that it is clear what they do.
Related: OS#6022
Change-Id: I395a60b2fba39bac4facec78989bac20f0cef0d3