dhcp-identifier: un-inline dhcp_validate_duid_len

dhcp-identifier: un-inline dhcp_validate_duid_len

After all it is used in more than one place and is not that short.
Also tweak the test a bit:
- do not check that duid_len > 0, because we want to allow unknown
  duid types, and there might be some which are fine with 0 length data,
  (also assert should not be called from library code),
- always check that duid_len <= MAX_DUID_LEN, because we could overwrite
  available buffer space otherwise.