davinci_emac: always build in CONFIG_OF code

clang warns about what seems to be an unintended use of an obscure C
language feature where a forward declaration of an array remains usable
when the final definition is never seen:

drivers/net/ethernet/ti/davinci_emac.c:1694:34: error: tentative array definition assumed to have one element [-Werror]
static const struct of_device_id davinci_emac_of_match[];

There is no harm in always enabling the device tree matching code here,
and it makes the code behave in a more conventional way aside from
avoiding the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>