Updated cellular debug trace prints:
- Removed unnecessary prints.
- Tracing more in DEBUG level.
- Read/write bytes not printed on big packets.
- Signal quality (RSSI) traced to log network problems.
- Dismissed AT data is traced.
- Modem type and firmware version are traced.
Network registration returns NotRegistered instead of StatusNotAvailable,
because that's not in 3GPP TS 27.007.
Missing CellularSocket constructor caused uninitialized values to be used.
Also changed comparision macros to ASSERT_EQ as suggested by GoogleTest guide.