net: dsa: felix: keep reference on entire tc-taprio config

In a future change we will need to remember the entire tc-taprio config
on all ports rather than just the base time, so use the
taprio_offload_get() helper function to replace ocelot_port->base_time
with ocelot_port->taprio.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>