OAI-UE-TDD hits subframe2harq_pid() = 255 and crashes

cause: UE detects a false DCI0 at subframe 5. When TDD UL-DL-config = 1, DCI0 can come to UE only at downlink subframe 1, 4, 6 and 9. The corresponding uplink subframe is undefined if DCI0 comes at subframe 0 or 5.

fix: Define a "mode" for DCI decoding. Decodes only uplink DCI when mode = UL_DCI; decodes only downlink DCI when mode = DL_DCI; decodes any DCI when mode = UL_DL_DCI; and skip DCI decoding if mode = NO_DCI.

Implement the [Table 8-2, 36.213] and [Table 4.2-2, 36.211]. Given a subframe selects a proper mode according to the tables.

For FDD the mode is always equal to UL_DL_DCI.