The dec_plmn function takes an hexstring and returns the decoded MCC and
MNC as integer values. The result is then used by the json encoder in
EF_PLMNsel, which means the json output will contrary to the input, use
integer values instead of strings.
This is not correct since there may be leading zeros (e.g. mnc 01 and
001 both exist are different) which must be retained in order to know
the correct length of the MNC.
Related: OS#4963
Change-Id: I393e04836814d992d2a6d0a4e4e01850976d6e81
6c5cd8031d