babeld: redo TS/PC check procedure for I-D rev 04

Revisions of the Babel authentication I-D prior to 04 required at least
one and allowed more than one TS/PC TLV in an authenticated packet (only
the 1st such TLV would be processed). Revision 04 will require that an
authenticated packet must contain exactly one TS/PC TLV. This change to
the code concurs with the change to the spec.