ussd: Reject and release unhandled SS requests/interrogation

In case the unpack of a USSD request is failing the channel would
remain open and the phone would not receive a response. Simply
reject the interrogation.

Example interrogation:
0000   1b 7b 1c 0d a1 0b 02 01 01 02 01 0e 30 03 04 01
0010   11 7f 01 00
1 file changed
tree: d41c6af95c7face71aca1da77031875e16d755d4
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README