net: gptp: fix rate_ratio in gptp_mi_set_ps_sync_cmss()

Setting a ration of 0 breaks the computation of scaled_rate_offset
in gptp_md_follow_up_prepare().

According to IEEE802.1AS 10.2.2.3.9, when sent by clock master
entity, rate_ratio shall be equal to gmRateRatio. But since
in gptp_md_follow_up_prepare(), in case we are the grand master
clock, we take the assumption that clock source is the local clock,
we may set this ratio to 1.0.

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