net: dsa: mv88e6060: prevent crash on an unused port

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: mv88e6060: prevent crash on an unused port

If the port isn't a CPU port nor a user port, 'cpu_dp'
is a null pointer and a crash happened on dereferencing
it in mv88e6060_setup_port():

[ 9.575872] Unable to handle kernel NULL pointer dereference at virtual address 00000014
...
[ 9.942216] mv88e6060_setup from dsa_register_switch+0x814/0xe84
[ 9.948616] dsa_register_switch from mdio_probe+0x2c/0x54
[ 9.954433] mdio_probe from really_probe.part.0+0x98/0x2a0
[ 9.960375] really_probe.part.0 from driver_probe_device+0x30/0x10c
[ 9.967029] driver_probe_device from __device_attach_driver+0xb8/0x13c
[ 9.973946] __device_attach_driver from bus_for_each_drv+0x90/0xe0
[ 9.980509] bus_for_each_drv from __device_attach+0x110/0x184
[ 9.986632] __device_attach from bus_probe_device+0x8c/0x94
[ 9.992577] bus_probe_device from deferred_probe_work_func+0x78/0xa8
[ 9.999311] deferred_probe_work_func from process_one_work+0x290/0x73c
[ 10.006292] process_one_work from worker_thread+0x30/0x4b8
[ 10.012155] worker_thread from kthread+0xd4/0x10c
[ 10.017238] kthread from ret_from_fork+0x14/0x3c