gNB: fix MAC TA command scheduling if measurement gaps are configured

In the current implementation, if UE transmission is interrupted via nr_mac_interrupt_ue_transmission(), UE_sched_ctrl.ta_frame is modified to be the previous frame avoiding TA scheduling in the measurement gap. So the next scheduled TA is shifted to be in 99 frames. As there are other measurement gaps before that, the scheduled TA is always shifted, but actually never reached, effectively disabling the MAC TA commands.

To fix this, UE_sched_ctrl.ta_frame is changed to not indicate the last TA command transmission, but the next one. This allows to check in nr_mac_interrupt_ue_transmission() if the next TA command transmission is within the measurement gap and if so shift it to just after that gap.