SMPP USSD Mica ussd_service_op codes are incorrect.

Currently the Mica ``ussd_service_op`` codes are incorrect. The list of supported op codes is incomplete and some of the existing codes confuse the hexidecimal and decimal values. The correct list of values should be:
```
1 (0x01) PSSR indication // start deliverSm -- BEGIN
2 (0x02) USSR request // sumbmitSm -- CONT
17 (0x11) PSSR response // sumbmitSm -- END
18 (0x12) USSR confirm // deliverSm -- CONT 
129 (0x81) USSREL indication // deliverSm -- ABORT
```
Sorry, I should actually explain my reasoning here.

At the moment, we work with inbound `ussd_service_op` values as decimal ints and outbound `ussd_service_op` values as hexadecimal strings -- this is confusing and we should use the same representation on both sides.