net: dsa: ocelot: fix selecting MFD_OCELOT

Commit 3d7316ac

 ("net: dsa: ocelot: add external ocelot switch
control") adds config NET_DSA_MSCC_OCELOT_EXT, which selects the
non-existing config MFD_OCELOT_CORE.

Replace this select with the intended and existing MFD_OCELOT.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/20230215104631.31568-1-lukas.bulwahn@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>