rename lchan->rqd_ta to last_ta

Originally, the lchan stored only the Timing Advance from the initial
channel request, hence it was called rqd_ta.

Since quite a while now, rqd_ta also stores the most recent Timing
Advance from each received Measurement Report. So rename to last_ta.

This is cosmetic preparation for an upcoming patch that clarifies
whether the Timing Advance is already known for Channel Activation.

Change-Id: I1049526a173819baeb4978db5bf018ba3f1006a0
6 files changed