ipa: Document ipa_ccm_idtag_parse_off and fix ipa_ccm_idtag_parse

ipa_ccm_idtag_parse_off is broken, and can only be used with
len_offset=1 on ID Request messages, otherwise won't work correctly.
Modify ipa_ccm_idtag_parse to at least parse those correctly, and
document the limitations.

Those two functions are already deprecated and only used in openbsc by 3
callers:
* ipa_ccm_idtag_parse in ussd_read_cb(): Broken, that function can only
work for Requests and it's used to parse a Response.
* ipa_ccm_idtag_parse_off in forward_sccp_to_msc (NAT): Broken, it can
only be used to parse Requests and it's used to parse a Response.
Furthermore, len_offset=2 is passed which makes no sense and most
probably it fails always, or can even make the program crash.
* ipa_ccm_idtag_parse_off in (answer_challenge): This one is fine and
could actually be replaced with ipa_ccm_id_get_parse after this commit
is merged.

Change-Id: I6efc852dfc041192f554e41a58290a0f63298021

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