net/smscx5xx: change to of_get_mac_address() eth_platform_get_mac_address()

Use more generic eth_platform_get_mac_address() which can get a MAC
address from other than DT platform specific sources too. Check if the
obtained address is valid.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>