net: dsa: ocelot: felix: Remove requirement for PCS in felix devices

Existing felix devices all have an initialized pcs array. Future devices
might not, so running a NULL check on the array before dereferencing it
will allow those future drivers to not crash at this point

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