net: enetc: remove of_device_is_available() handling

Since commit 6fffbc7a ("PCI: Honor firmware's device disabled
status"), this is redundant and does nothing, because enetc_pf_probe()
no longer even gets called.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>