Some modems return the +COPS operator name in hex-encoded current
character set (as set with +CSCS). Others return the operator name
in ASCII when set to UCS2, while yet others return the ASCII name
with trash at the end (*cough* Huawei *cough*). Handle that better
by not crashing.
This method will try to convert the input string to UTF-8. The input string is
supposed to be in the given charset; or otherwise is supposed to be the hex
representation of the string in the given charset.
Change interface to take the number of GSM characters
rather than the number of octets, so that it is possible to
distinguish the 7-character and 8-character cases.