Minor fixes for SMSCB

These fixes were necessary for me to get SMSCB working properly. Props to Chuck Bokath and Donald Kirker for the discussion on OpenBTS-Discuss that led to these changes. 

Changed hardcoded language code in SMSCB.cpp to 0x01, which is English in the default GSM encoding, which fixes some display errors, especially on Android phones. 

The fourth L2 CBCH frame in an SMSCB message now has the Last Block Bit properly set. 

GSMTAP now properly reports SMSCB messages, and Wireshark shows the messages as expected.