Use qbit-TA to update Timing Advance

Separate qbit-TA to TA conversion into separate function and use it for
computing and updating Timing Advance.

Note: the code was tested with TA=0 only to make sure it does not
introduce regressions.

Change-Id: I96fdbb20b09fb85fdd9fb6dcf3c25f6bee7f80e4
Fixes: OS#1531
5 files changed