bugfix: fix timing advance

- remove global variables in the function lte_est_timing_advance_pusch
- do the smoothing logic that was in lte_est_timing_advance_pusch
  in rx_sdu (is it necessary? is it correct?)
- put back the value 100 in prach_procedures for the test on prach energy
- change the value of timing_advance_update

  it was:

     timing_advance_update                  = sync_pos - eNB->frame_parms.nb_prefix_samples/4; //to check

  it is now:

     timing_advance_update                  = sync_pos; // - eNB->frame_parms.nb_prefix_samples/4; //to check

  this should be checked somehow. The computation looked suspicious. Maybe
  the new one is wrong.