dhcp-identifier: handle too long duid_len in dhcp_validate_duid_len()

dhcp-identifier: handle too long duid_len in dhcp_validate_duid_len()

Callers of dhcp_validate_duid_len() know that they must not pass
a zero duid_len. Thus asserting against that is appropriate.
On the other hand, they are not aware of the maximum allowed length
of a duid, as that is the reason why they call dhcp_validate_duid_len()
in the first place. So dhcp_validate_duid_len() should just signal a
regular error.

Thereby, change assert_return() to an assert() as this is an internal
function.