GSM SMS: fix dissection of Character Set IE

Also do not display a 8bits data SMS as an expert error
Those 2 points led me to a wrong analysis yesterday

Change-Id: Ib319e2ef8f0e7297cd2e119b6d0d98638e6a3d61
Reviewed-on: https://code.wireshark.org/review/5763
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>