fixup pcu/GPRS_Components: do not duplicate existing functions

Both f_ms_rx_imm_ass_ccch() and f_ms_establish_ul_tbf() functions
are actually twin brothers of good old f_pcuif_rx_imm_ass() and
f_establish_tbf() with some minor changes.

The former accepts a GprsMS parameter, that is never used. The
latter simply calls f_ultbf_new_from_rr_imm_ass() in the end.

Let's avoid code duplication:

  - call f_establish_tbf() from f_ms_establish_ul_tbf(),
  - remove f_ms_rx_imm_ass_ccch(), use f_pcuif_rx_imm_ass().

After the removal of f_ms_rx_imm_ass_ccch(), the implementation
of TC_ta_idle_dl_tbf_ass() does not need the GprsMS state, so
let's make it look like it was before.

Change-Id: If6c0b8796500e96525b7b1cadb61ab2fc84b4744
2 files changed