enc28j60: Use device_get_mac_address()

Replace the DT-specific of_get_mac_address() function with
device_get_mac_address, which works on both DT and ACPI platforms.
This change makes it easier to add ACPI support.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>