net: mscc: ocelot: delete unused ocelot_set_cpu_port prototype

This is a leftover of commit 69df578c ("net: mscc: ocelot: eliminate
confusion between CPU and NPI port") which renamed that function.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>