net: gptp: Fix announce message len

This fix addresses wrong announce message length warning message.

TLV is a variable length (4+8N) based on the 802.1AS-2011 (table 10-8). In
Zephyr TLV is fixed to 12 bytes. TLV type and length are already taken
into account in the announcement message length.

Signed-off-by: Mario Paja <mario.paja@zal.aero>