net: gptp: fix again computation of follow up correction field

Commit 413eceed ("net: gptp: fix follow_up message correction_field")
breaks computation of follow up correction field in case time aware system
is not the grand master clock but behave as a transparent clock which relay
sync and follow-up messages from one slave port to one or several master
port(s). In this case, according to IEEE802.AS 11.1.3 and 11.2.14.2.3,
md sync send state machine shall compute the sum of link propagation delay
and residence time, expressed in grand master time base. Then this quantity
shall be added to last received fup correction field to build value of
correction field. This patch checks whether time aware system is the grand
master clock or not, and computes value correction field accordingly.

Signed-off-by: Jean-Nicolas Graux <jean-nicolas.graux@st.com>