Bugfix to TX_DATA.request PDU_Length computation, change input parameter to uint32_t

Bugfix for TX_DATA.request PDU Length calculation, changes input parameter of compute_PDU_Length to expect 32 bits. The total_length parameter in compute_PDU_Length being uint16_t causes the passed value to be possibly truncated, which can cause the PDU_Length to be incorrectly calculated. This change prevents that truncation, since it now matches the size of the values passed into the function where it is being called.