net: dsa: pass CPU port fixed PHY to .port_disable

While adding the logic for DSA to register a fixed-link PHY for the CPU
port, I forgot to pass it to the .port_disable method too, just
.port_enable.

Bug had no impact for felix_switch.c, due to the phy argument not being
used, but ksz9477.c does use it => NULL pointer dereference.

Fixes: fc054d56 ("net: Introduce DSA class for Ethernet switches")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>