niu: always include of_device.h

The niu driver uses struct of_device when built on any arch, not
only SPARC64, so always #include <linux/of_device.h>.

drivers/net/niu.c:9700: warning: 'struct of_device' declared inside parameter list
drivers/net/niu.c:9700: warning: its scope is only this definition or declaration, which is probably not what you want
drivers/net/niu.c:9716: warning: assignment from incompatible pointer type

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Dave S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>