niu: introduce temp variables to avoid sparse warnings when swapping in-situ

Suppress a large block of warnings like:
drivers/net/niu.c:7094:38: warning: incorrect type in assignment (different base types)
drivers/net/niu.c:7094:38:    expected restricted __be32 [usertype] ip4src
drivers/net/niu.c:7094:38:    got unsigned long long
drivers/net/niu.c:7104:17: warning: cast from restricted __be32
...

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>