[ANCP] Range of values of the Code field

[Tom Taylor <tom111.taylor at bell.net>]

The IANA registry for the GSMPv3 failure codes currently implies that the range 
of values is restricted to 8 bits. However, we have defined the field to occupy 
12 bits. A simple solution is to add the specification that the range of values 
from 256 to 4095 is reserved for IETF use.

Tom