PDU Length is as per SCF 222.10.02: The total length (in bytes) of the
PDU description and  PDU data, without the padding bytes. From
TX_DATA.request we get 8 (2 bytes PDU_Length + 2 bytes PDU_Index + 4
bytes num_TLV ) and from each TLV we get 4 + value size ( 2 bytes tag +
2 bytes length + value size without pading)

Therefore, add function to compute PDU_length() value for
TX_Data.request, taking into account how many TLVs there are.
compute_PDU_length() does not rely on access to `nfapi_nr_pdu_t`.
Put it into nr_mac_common.c to avoid introducing dependencies.