niu: Use resource_size instead of private function

The function res_size in drivers/net/niu.c is a copy of resource_size in
linux/ioport.h. Remove the function and use resource_size instead.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>