net: gptp: do not run clock master sync send SM if we are not the GM clock

Without this patch, in case time aware system is grand master capable
but is not the grand master clock and behave as a bridge so that it
shall ONLY relay SYNC and FUP messages from slave port to master port(s),
SYNC and FUP are mistakenly generated by ClockMasterSyncSend state
machine.

This patch only call ClockMasterSyncSend state machine in case a Grand
Master clock is present and is this time aware system.
This check is not described by IEEE802.1AS. Instead, according to
10.2.9.3, the SiteSyncSync state machine shall not take into account
information from ClockMasterSyncSend in case this time aware system is
not grand-master capable. Current implementation of ClockMasterSyncSend
state machine sends sync indication to the PortSync entities, instead of
sending it to the SiteSyncSync entity. And the SiteSyncSync state machine
does not make any check.

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