The TLV_IE.from_tlv() method is part of a base class that is inherited
by more specific classes. The official way to obtain the tag is the
inherited-class-provided self._compute_tag() method, and *not* a direct
reference to the self.tag member.
This allows for some more obscure TLV parsers, such as the upcoming one
for Proactive Commands in the CAT/OTA context.
Change-Id: I0cd70e31567edc5a0584336efcb5e4282734f6dd