net: qcom/emac: 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: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>