net: sxgbe: Fix NULL dereferece when using DT

When the MAC address is provided in the device tree file, the
condition is true and kernel crashes due to NULL dereference.

Signed-off-by: Girish K.S <ks.giri@samsung.com>
Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>