realtek: ignore disabled switch ports

realtek: ignore disabled switch ports

When marking a switch port as disabled in the device tree, by using
'status = "disabled";', the switch driver fails on boot, causing a
restart:

    CPU 0 Unable to handle kernel paging request at virtual address
    00000000, epc == 802c3064, ra == 8022b4b4
        [ ... ]
    Call Trace:
    [<802c3064>] strlen+0x0/0x2c
    [<8022b4b4>] start_creating.part.0+0x78/0x194
    [<8022bd3c>] debugfs_create_dir+0x44/0x1c0
    [<80396dfc>] rtl838x_dbgfs_port_init+0x54/0x258
    [<80397508>] rtl838x_dbgfs_init+0xe0/0x56c

This is caused by the DSA subsystem (mostly) ignoring the port, while
rtl83xx_mdio_probe() still extracts some details on this disabled port
from the device tree, resulting in the usage of a NULL pointer where a
port name is expected.

By not probing ignoring disabled ports, no attempt is made to create a
debugfs directory later. The device then boots as expected without the
disabled port.

Signed-off-by: Sander Vanheule <sander@svanheule.net>