Change PDU_Length and tlv length calculation to be inline with SCF 222.10.02

According to SCF222.10.02, the PDU Length parameter of the TX_DATA.request message is described as being the following: "The total length (in bytes) of the PDU description and PDU data, without the padding bytes." (SCF222.10.02 Table 3-58). This MR changes the value calculated for the PDU_Length to be in accordance to this: 2 bytes PDU_Length + 2 bytes PDU_Index + 4 bytes num_TLV + TLV size ( 2 bytes tag + 2 bytes length + value size without padding), which when only 1 TLV is added, is the tlv payload length + 12.