net: dsa: mv88e6xxx: group cycle counter coefficients

Instead of having them as individual fields in ptp_ops, wrap the
coefficients in a separate struct so they can be referenced together.

Fixes: de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: Shenghao Yang <me@shenghaoyang.info>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>